The 400 has seventeen more cubic inches than a 383. And a larger bore with less valve shrouding. With the same CR, cam, heads, intake and exhaust, the 400 will make more horsepower. I think the 400 block is also stronger.
400s are like 360s. They got a bad rap because stock they were all low compression smog motors. Build them the same as the smaller motors and the 400 is stronger than a 383, and a 360 is stronger than a 340. And a 440 is stronger than a 426 which is stronger than a 413. Again, this is assuming everything except the cubic inches is the same.