anyone know if a 400 from a 1979 trans-am the same size as 350 from a 68?if yes,will it be a hassle to change motor mounts etc?

All modern Pontiac blocks are pretty much the same externally. I.e., no "big" block vs "small" block like Ford or Chebby.

Your issue will be motor mount mounting holes cast and/or drilled into the block. Our FGF's use a 2 hole setup. 2nd gen Firebirds and later Pontiacs utilized a different 3 hole arrangement.

In order to be compatable, mid-'70's blocks had a 5 hole arrangement that was compatable with the early 2-hole and later 3-hole. Pontiac phased out the 5-hole arrangement in about 1976.

However, Firebird repro parts houses usually sell an addapter that will help putting later 3-hole blocks into FGF's.

There is more info on engine swaps in the FAQ's under Engine, General.

<Pontiac phased out the 5-hole arrangement in about 1976.>

It seems to be an unauthorized change, meaning that it was left out by some sorry pos supervisor who didn't give a chit about building a quality part that interchanges. Myself and others have had post '77 blocks that were 5-bolt drilled.

Keep in mind that the low compression '79 400 may have less power than your '68 350 in stock form.