Any listing for an engine that indicates that it has a one piece rear seal has a different flywheel bolt pattern. So your early flywheel will not bolt up. There are also some other differences. Some of the later blocks are designed to use a block mounted fuel pump others do not. Some (as indicated in Jegs specs) use the early heads with the perimeter bolt pattern on the newer engines.
Others use the center bolt heads which require a completely different intake manifold from the early engines.
It's very confusing even for those of us that have been around this stuff for some time. The good thing about Chevy engines is that parts interchange but finding the right combination has been made more difficult lately.
Fred
