[Redog]

The 400 tranny will work, but you'll need an adapter plate to hook it up to the motor. BPO have a differnet bolt pattern than Chevys

Get a good 350 or 455 (350, 395558 "2" block from 68 to 72 cast iron and soild mains, maybe somebody else can shead light on the best 455) and basicly a good cam, lifters, decent manifold, decent ign, good heads (stock 5 or 7a for small blocks and C for the big block) and a set of headers and you'll have a pretty decent streetalbe motor. Just be sure to get good cooling, and a deeper oil pan would be a good idea too!