small block to big block chevy

I have a 89 chevy k2500 with a 350 and a 700r4 tranny. i want to swap my 350 to a older model 396, but i wnat to keep my tranny. what do i need to make that happen?  

It's the same bolt pattern. It'll match up fine...I gotta ask, why the swap?  

Going to need to beef up tranny and change axle gears. My guess is you want to do this because you are lacking power with current configuration of axle gears and tires but if you plan to try to muscle your way thru this with a 396 you will cook tranny. Also be advised that no 396 had hardened valves and seats for unleaded gas so you need to upgrade them or they will not last long under hard usage.  

i want the big block power,  

my tranny was just rebuilt with better parts and set up for towing and the power. but the reason i asked them questions is because i was told that because of the computer. i was told you cant do it with out a special chip.  

A 700R4 needs no computer to work. That is beauty of it. But again I do not care if tranny has been beefed up if you have tall gears for tire size you will fry tranny riding the converter stall with extra torque. At this stage before you do swap (because that 350 TBI is not a weak motor and responds well to better fuel and advanced timing) is to invest some time and money in some new axle gears and you may find that you do not need a 396 and not have to worry about tranny cooking either. Take your tire size and divide it by 7.2 and that will give you the axle ratio you need here with a 700R4.  

well what tire size and gear ratios are you running??? i don't like jumping to conlcusions about too tall axle gears cuz sometimes wanting big block power is a legitamate answer  
solid axle chevys cant be beat
78 K5: 350 V8,700r4,np208, 8 lug D44 4:88, 14 bolt 10.5 4:88 with detroit locker and 35 inc parenelli jones dirt grip tires

I suspect that it is a cover/attempted fix for big tires and tall gears. Big clue here is that tranny has been rebuilt at least once already.  

i am running 31 10.5 now and not sure of the gears. i think there stock. but i plan on running 35 14.5 and going to 410 gears.  

if it needs no computer to work what about the plug on the tranny. or should i leave my computer in and just diconnect the motor part. i am running 31 10.5 but going to be running 35 14.5 tires. my gears are stock but i am upgrading to 410. i have no converter stall now and dont plan on using one. stock converter is fine by me. i am goin to be doin alot of towing my mud boggers with it.  

okay 4:10 gears for a towing truck with 35 inc tires is not gonna cut it.... you need at least 4:56 but since your gonna be towing go with 4:88 remember you have overdrive, the 700r4 dons't use computer, the plug on the side of tranny is just for the tourqe convertor lockup operation which is controlled by okay maybe you could call it a computer but a really simple one in a stock one, i swapped in an 87 700r4 into my 78 blazer and i have no computer but tourqe converter lockup still works cuz of aftermarket lockup kit which is nothing more than a vaccum sensor and little stuff, no computer.  but seriously dont put in 4:10s cuz your gonna hate it, you will kill the tranny right away and you won't be using all the potentiol the 396 has, at least 4:56 but id go with 4:88  
solid axle chevys cant be beat
78 K5: 350 V8,700r4,np208, 8 lug D44 4:88, 14 bolt 10.5 4:88 with detroit locker and 35 inc parenelli jones dirt grip tires

4.10 gears with 35's and a 700R4 IS NOT going to cut it in any scenario and will lead to early tranny failure. (some are just so hung up of this 4.10 thing being perfect and also forget OD changes the game too) You want 4.88's with 35's as 700R4 has a 30% OD gear. What this means is that at 65 MPH you would only be turning 2100 RPM with this 35/4.88 combo. I would not suggest/support a lower cruise RPM with this combo of tires and speed. I would use same ratio towing I just would not use OD towing. If you live/travel regularly in mountains and higher elevations (above 4000 feet or so) I would consider a 5.13 instead as engine power drops at least 3% per thousand feet in elevation and becomes quite noticeable above 5000 feet or so.  

i jus say put in the 396 cuz they are great motors im currently runnin one and if ya fry the tranny you fry the tranny  

if you are upgrading to a big block i would suggest upgrading to a 350 or a 400 turbo tranny they can withstand up to 450 and 550 hp stock and you can get them pretty cheap at any salvage yard and on craigs list that way you wouldnt waste any money on new gears and you wouldnt risk burning up your current tranny or your new big block and by the way turbo 400's make great towing trannies  

454BBC:
if you are upgrading to a big block i would suggest upgrading to a 350 or a 400 turbo tranny they can withstand up to 450 and 550 hp stock and you can get them pretty cheap at any salvage yard and on craigs list that way you wouldnt waste any money on new gears and you wouldnt risk burning up your current tranny or your new big block and by the way turbo 400's make great towing trannies

Your kidding right? A well built THM 350 can handle that regulalry as can a warmed up THM400 but not stock. Also a properly built 700R4 can handle about 700HP so if you build it why build old technology and just 3 gears??? Also there is no honor in cooking a tranny as it is better to do it right first time.