Make sure the 400 block has the same holes for the motor mounts that the 350 does. Pontiac used two different bolt patterns, and not all will interchange without adapters. Some have both patterns, so you might get lucky.
400's & 350's have the same stroke, 3.75". Pontiac crankshaft flanges changed a couple of times in the '60s & '70s, so be sure you have a flywheel/flexplate with a center hole that matches the crank on the 400.