Converting from a PG to a TH400 does the crossmember need to be changed? When pairing up the 2 transmissions it looks like the TH400 is slightly longer than the PG. Will the TH400 fit without changing anything?

The 400 and the 350 crossmembers are different. The 350 crossmember fits the 350, PG and manual trannys.

You also have to change your drive shaft, and yolk, and the kickdown (to a cable).

TH400 uses an electric kick-down.

Ya, you're right, I guess it is electric, I couldn't remember all the facts, check out this link (from this website), it tells you everything, just scroll down to the 400 conversion.

https://firstgenfirebird.org/FAQ/trans_clutch/trans.html

I read the F.A.Q. about the trans swap. It doesn't specify if you have a long tail or short tail transmission. I have a TH-400 with a short tail. There is only about an inch different in length between my PG and TH-400. I understand I have to change the yoke, but do I really need to change the shaft too?

You can take your drive shaft to a shop and have it shortened. I had mine balanced this past winter, with very good results.