The c 60 series trucks have a deep oil pan on the trans from the factory and they have more clutch plates and something else that i can not think of wright now i think it is something to do with the amount of springs in side i forget.
Lonestar is dead on. The B O P bolt pattern is for old buick and pontiac which is not the same pattern as chev and gmc engines. There were some 200r4's made that had dual bolt patterns but only some. There were no 400's that had dual bolt patterns. In other words if you want to put a chev engine in it you need a tranny with a chev bolt pattern. The engine mounts will also be different.