Besides the seal what else is different in a 1 pc rear seal motor vs 2pc rear seal motors? ( non roller cam type)

If I get an older motor Do I have to get a different starter and flex plate to make it fit my '88 th400 transmission? Will my old torque converter still bolt up?

Aslo will my pre-87 heads and intake manifold fit?

doing the same thing in reverse 73 to 92. yes flexplate and starter are different. my new 168 tooth flexplate bolts to my th400 same as old 153. and yes heads and intake will fit.

Starter goes with the size of the flexplate (168 or 153 tooth), not the rear main seal type. If you used a 153 tooth flexplate on the later motor and put a 153 tooth flexplate on the older motor, your starter from the later motor would work on the older motor (as long as it wasn't so old that the starter mount holes aren't drilled properly).