Let's start at the beginning. First, the Pontiac and Chevy motors are completely different designs with different bore, stroke, castings, and bolt patterns. The motor mounts are different. The bellhousing bolt patterns are different. The accessory brackets and locations are different. The exhaust manifolds are different. The starter motors are different and are on opposite sides. The only thing they have in common is the distributor cap!
The Chevy 400 is in the small block family. Pontiac motors have all one family (excepting 301s and 267s with their shorter deck height) so there is no big/small block distinction.
As for the trans, the BELLHOUSINGS are different, but with the correct bellhousing, you can bolt any GM-compatible trans to either motor.