Termite53, a very good friend of mine recently pulled a Gen 1 350 sbc from his '32 Ford roadster and replaced it with an LS1. The engine itself was a bolt in with some simple plates from Speedway Motors to adapt the side engine mount locations. The time, effort and money went into the electronics and wire harness.
The LS1 bolted right up to a TH400 and he used a "switch pitch" convertor for some reason I've forgotten.
If you like, I could put you in touch with him. He'd tell you whether or not he'd do it all again or stay old school, given a choice. My email is
[email protected].