Is the Pontiac 400 engine the same block as the 455 engine with a different stroke and bore?

Basicly the same design, Yes, The crankshafts do not interchange though since the 455 has 3.25" main journals and the 400 has 3.00" mains. Heads, intakes, cams, oil pans, oil pumps, distributors, ect all interchange though. 400 is 4.12 bore 3.75 stroke 455 is 4.15 bore 4.21 stroke