Need to have main journals turned to 350 size and you will need 400SB rods and then use 350 pistons. There is a slight change you may have to debur crankcase throw area for counter weight clearance. (if you need to do this it is minor) Also if you plan to run only 93 octane, shoot for 10 to one or a bit better CR and maybe a 268 instead of a 260. (as you increase CR you can run a bit more duration and still get a good idle)  

now I see the problem,
the 377 I built is a 400 block (4.125-4.155) with an internally ballanced 350 crank (3.48) using all off the shelf parts and without ruining the coating on the crankshaft by turning them down to fit a 350 block, which is what most people refer to as a 377 also it makes more hp and  slighty less torque than your version of a 377 which is what we all call a 383. most of the time when building an engine your lucky if you can get away with not doing some boring to get rid of the cylinder taper and or some scaring. I also run between 260 and 268 duration, 258 for a low rpm monster, although 268 and 270 is pushing it unless you're building an engine for a car not a 4x4. gearing only takes you so far when you also intend to efficiently keep up with traffic on the hwy  

the 383 or if we must, 377 stock bore with a 3.75 stroke
is by far the most popular stroker, there are kits available already ballanced and ready to get stuffed into a 350 block, or you can buy a new GM performance crate 383 with a healthy warranty and awesome hp/torque numbers. (340/435)
an engine builder talking about a 377 isn't refering to a 3.75 stroke
one more thing.......

350 rods are 5.7"..........400 rods are?????....  

Mud is confused here. A 377 is a 350 block with stock bore and a 400 crank with journals turned down. A 372 is a 400 block with a stock bore and a 350 crank with special bearing inserts. A 400 SB bored 30 over with a 350 crank is a 377 and a 350 bored 30 over with a 400 crank is a between a 380 and a 381 for take your pick. (bored 40 over it is a 384) Once again the crate motor 383 uses a non standard stroke of 3.8 so you will not be able to play mix and match on a rebuild like you can with a 377 based on a 350 block with a off the shelf 400 crank turned down and stock 400 rods and stock 350 pistons. (BTW, you can use longer 350 rods with this swap which will give it more top end but you will need special pistons with a different compression height because of longer rods.  

not confused at all.....sorry. as you just pointed out.  

Look at GM performance parts dot com,i saw an link to put together a engine and dyno it ,page i saw was kinda limited in scope though, but things change.  

there also is a reason why builders and kits don't use stock 400 rods.
With that long of a stroke the stock rods have a poor rod/stroke ratio. Even 5.7's don't help that much. The longer rod reduces the rod angularity, which reduces the side loading on the piston, which in turn reduces the friction (free HP) and also reduces the tendency to oval the bore. Another claimed benefit to the longer rod is that it allows the piston to dwell at TDC longer, allowing the flame front to travel across more of the piston before it starts down the bore. all of which actually make the crate engine the best choice, some use the 400 crank and regrind it to a small journal 3.8 stroke, even more desireable.
the 5.56" long 400 rods have a tendency to cause a few problems like premature wear and broken piston skirts.  

Unless you are taking it to 6000 RPM and more a lot, the rod length is not a issue. There is no need at all for after market rods for a engine build up for a 4x4 unless you are truly racing and running past 6000 RPM all the time with this stroker otherwise it is wasted money. The beauty of a SB is that you can make one run good with stock parts and that is why they got to be so popular over the years. ALso premature skirt wear is more a factor of not change oil enough or using right kind more than anything else. For some reason many skip on oil changes even though it is your engines lifes blood.  

that is the reason for using stock 350 5.7" rods over the 400 rods, rod length is an issue, the 400 is the worst of all sbc's for skirt and cylinder wear no matter how they are/were taken care of, there are people here that value your infomation and opinion including myself, so when you venture into something of this nature be sure you know what you're talking about, you always preach the right way and no short cuts, using a 5.56" rod when the correct application is available is not right, this is what I did before I got greedy and started driving, my shop is full of machine equiptment.

the longer rod changes the position of the ristpin in the piston. when you do a .020-.030 bore you buy new pistons anyhow.

the 400 rod in the 383 is no different than me running 4:10 gears with 36" tires.  

I have seen some 400 SB's go 300K as long as they were properly serviced and never overheated. Like I said sure rod angle is a issue but again it is not as much as you make it out to be. When you do not change your oil often enough the grit in it build up and accelerates wear on bore and skirts from side thrusts and the gritty oil is squeezed between them. A oil with better rod angle will tolerate this a bit better but if you use good oil and change it every 3K or less you will not find it to be a problem at all. One more thing, if you are looking for good low end torque, short rods will give you that. Longer rods favor higher RPM efficiency. I have been messing with SB for over 35 years and I know what works and why. Just like some just have to have a 4 bolt main block but again if you are not taking it over 6 grand regularly or using a super charger a 2 bolt block will do fine. Also cast iron cranks do well to with this range too and actually tend to wear better because by nature they tend to be straighter and run truer because they have less internal stress built up in them that causes them to go out of true vs forged units. It is all about what you really need verse what some think you need.  

the short rods rob torque not make more, it's a geometry and friction issue from the rod angle, why would you build an engine that will self destruct and limit yourself to 5,000 rpm, when for the same cost you can do it right, even when rebuilding a 400 we use 5.7 and 6." rods to improve the bottom end torque, horsepower and service life.
all this hi pro talk, then you insert a cast crank and insuficient rods to top it off, we aren't pushing snow, we go fast and hard.

lets just all over build our differentials and gears then put a poorly engineered engine to try and break them.  

mudb8-.:
the short rods rob torque not make more, it's a geometry and friction issue from the rod angle, why would you build an engine that will self destruct and limit yourself to 5,000 rpm, when for the same cost you can do it right, even when rebuilding a 400 we use 5.7 and 6." rods to improve the bottom end torque, horsepower and service life.
all this hi pro talk, then you insert a cast crank and insuficient rods to top it off, we aren't pushing snow, we go fast and hard.

lets just all over build our differentials and gears then put a poorly engineered engine to try and break them.

Again you are in error here. Shorter rods do make more torque because while they increase side thrust forces on piston they also increase the thrust placed on rod journal relative to bore pressure at times of peak pressure and at lower RPM's and this together with a quicker expansion rate for shorter rods  (piston accelerates faster off of TDC) increases usable torque. At higher rpm this is lost because it has to do with rate of mixture expansion and piston acceleration with a shorter rod. This is why you use shorter rods for better low end and longer rods for better top end (say 4000 RPM plus). This is engine design 101 dealing with basic physics involved here. You must have missed this "class".  

I think of a rod as a lever, a longer one will will have more mechanical advantage to move X. So i also think that a crank with a longer journal throw  has a greater circumference ,so a greater potential for mechanical advantage than a smaller circle, so if the piston is at or near tdc longer then fuel has a chance to more completely combust and exert greater force longer, wouldnt a long rod,long stroke engine be ideal?  

you should have paid attention better during the rod/stroke ratio part of the class, we aren't playing with a 3" stroke.  

wow i go away for a day and miss all the excitement. i dont really know enough on this subject to weigh in but i was always told long rods = torque and short rods = top end horse power. but i really dont know for sure.