What is the difference between a Chevy 400 and a Pontiac 400?

Basicly everything, The Chevy 400 is a member of the small block Chevy engine family and has a great deal of parts interchange with 265-350 chevys. (heads, intakes, distributors, water pumps, exhaust manifolds). The small block Chevy 400 has 4.40" bore spacing, a deck height of 9.025 and 2.65" main journals.

The Pontiac 400 is a member of the Pontiac V8 family and has interchangability with Pontiac V8s 287-455. (heads, intakes, waterpumps, distributors, exhaust manifolds). The Pontiac 400 is physically larger with a bore center of 4.62", a deck height of 10.23" and main bearing journals of 3.00".

The two engine families however have almost nothing in common, and almost no parts will interchange. The only thing they do have in common is the both share a 4.12 bore and 3.75 stroke. Pistons, crankshafts, and connecting rods, will NOT interchange due to design parameters. Both engines also require a specific transmission bellhousing since they do not share the same bolt pattern.