Is the auto tranny flywheel smaller than the manual tranny flywheel (starter gear teeth area)? And is a 1990 NA tranny the same as a 1986 NA tranny?

 

I think the diameter is the same. You can't use a clutch with the auto tranny flex plate because it doesn't have a friction surface and it doesn't have holes to bolt a pressure plate to it.

The trannys will fit the engine, but the crossmember mount holes for a series 4 and series 5 tranny are slightly different.