is there a way to tell or look up if my 80-85 350 4 bolt main is a 1 piece or a 2 piece rear seal?

Anything before 87 was 2 piece. I think 87 was the changeover year but 87 might be a mixture.

on that note, other than the flexplate, what's different from a 700r4 that's bolted to a 2pc rear main seal and a 700r4 that's bolted to a 1pc rear main seal?

you may wanna double check me on this but i belive the pre 87 had a 27 spline and post 87 had the better 30 spline