Some things to consider:

It is getting increasingly more difficult to find a good 400 block, and they are much more prone to cracks than a 350 block. A 2-bolt main 400 block is stronger than a 4-bolt main block. Don't be surprised if the first couple of blocks you find will be unusable.

The cost of building a 400 or 383 is about the same. But you will have to drill steam holes in whatever heads you use on a 400. People that don't, or fail to use a 400 head gasket, will have cooling issues.

It is better to use 5.7 or 6 inch connecting rods in a 400 as there will be less side load on the cylinders, and you can get a more efficient combustion chamber burn with the longer rods. For ultimate performance Smokey Yunick recommended stuffing the longer connecting rod you can fit into an engine.