I am in the process of swapping my 305 TPI to a Vortec 350 that was pulled froma 98 suburban(but it has a few tasty mods) The question I have is, what flexplate do i need to bolt my motor to the 700R4 trans? And, is this motor (the vortec) internally or externally balanced?

I think I need a 153 tooth one piece rear seal flexplate...but just wanted to confirm my thoughts.

The flexplate end is externally balanced and the harmonic balancer end is internally balaced. The same as a 1987 up 305 TPI. If your is a 1987 up you can use your 305 flexplate and starter. If not get one for a L03/LB9/L98 and you will be fine.

so I need an externally balanced 153 tooth flexplate? Like this one?

GM Performance Parts Flexplates: NAL-14088765 - summitracing.com

edit: my car is an 86...

Actually, I think that the 1 piece rear seal started as early as 1986 in some applications. You may not even need a different flexplate.

I am trading my motor for the vortec, and he needs the flexplate, which is the reason I'm asking if it's different

edit: the linked one is correct?

The vortec uses a 168 tooth externally balanced flexplate. The TPI should have a 153 tooth extrenally balanced flexplate (if 1 piece rear seal equiped).

sorry, my wording was confusing

What I mean is, can I bolt a 153 externally balanced to the vortec so i don't need a new starter, like the one in the link a few posts up

Yes, 100% interchangeable as long as it is 1 piece rear seal. A 2 piece rear seal flexplate is completely different.

thanks