i have a sbc 400 cast #3951509 i have heard a lot about 509 casting  is that the last # on the back drivers side of the block  or is 509  some where else on the block ?any advise on theis issue would be helpful  i am starting to rebuild it and would like as much knowledge about it as possible thanks.  

There was only two casting numbers for that block. A 3951509 was made in two batches. First was from 70 to 71 in 4 bolt main style. Second batch was from 74 to 80 in 2 bolt style, There was also a 3951511 made from 70 to 73 that could be either 2 bolt or 4 bolt. Given that the "509" had a longer production,run it should be the better block. You do not want to bore these motor out much because cylinder wall castings are kinda thin. GM limit is .030 but I would use a .020 limit if I was boring one out. Boring not only reduces wall thickness but also reduces the head gasket contact area between the joined cylinders so you want to keep a lot of meat there. One more tip. You never want to overheat a 400SB if you want it to last. It is pretty trouble free if you keep it cool. Make sure you have a big radiator and cooling fan up to task in any scenario. Cook these a time or two and you can crack block, crack head or at very least blow a head gasket. Further more, keep RPM down and realistically limit it to 5000 RPM or so. Though you can go a little higher, they can start coming apart above 6000 or so with stock parts in piston and rod area.  

thanks for the info  

snoman is right on the overheating part. on our most recent trip my step dad overheated his 400 sb by forgeting to turn on his fans after he just got done telling me not to forget. lol carma i think. in the hills and probally cracked a head but im not sure as we hav not tore it apart yet.  

I would NEVER use electric fans only to cool any V8. They are a poor sub for a good engine driven fan.  

obviously you havnt wheeled in the deep mud, because there is nothing worse then sticken your truck in a hole where water comes in the engine compartment and slings mud all over then when you have to do trail work....not fun.  

Actually I have. I would suggest a electrically actuated clutch fan. ALso in you properly shroud radiator it will not suck up mud and water from below and you could even install a winter cover bra when you are running in deep water to keep it from pushing through front. The point is that electric fans do not have the capacity to cool the engine all the time. To do so they would need to use a few hundred amps at minimum. A engine driven fan can move gobs of air in amounts that electric fans can only dream of. Somewhere I posted a article about how much airflow was need to cool a large motor running with a load and the amount of HP needed to do it. (maybe here??) It is far beyond the ability of aftermarket electric fans. To me, proper engine cooling in any scenario takes priority over every thing else. You over heat you "die"  

Whatever  

you talk your technical talk but it boils down to real world senario's and in our world the electric fan have done just fine in inyense situation and so have 4.11 gears, a 4 speed, and guess what? 40" tires.