would and old school 383 a 350 2 piece rear main seal block with a 400 crank etc. be considered internally balanced or externally?? would you use a 400 or a 350 flexplate??? thanx

i think external balance.
external factory balance 400-flexplate.
check the back side of the harmonic balancer for the offset balance design.

I have an old-school Stroker in my '59 Impala and it's externally balanced with a 400 flexplate, it's a '74 350 4-bolt core.

383 is externally balanced but, I think eagle makes an internally balanced crank for that.

I used a scat internal balance crank

I would have to say that it depends on the kit you go with. You can get an internal or external kit.

If you do it the "old-school" way, which is to take a stock 400 crank, grind the mains down to 350 size, and use stock 400 rods and stock .030" over 350 pistons, then it will be externally balanced. The way we all used to do it in the early 80s, when it was strictly a one-off kind of thing. You would use a stock 400 crank damper and a stock 400 flex plate or flywheel.

The "kits" are the "new school" way of doing it.