Personally, I like the 400 block as it leaves more room in the engine compartment and for all the world a 500" low-deck looks like a 383; guess that's the old secretive downplayer gene in me. My last 2 BBMs were 440 blocks in early B bodies, and they were a pain to work on on the right side.

Another thing I like about the 400 block is the bigger cylinders help unshroud the valves/flow a bit.

Your situation may well differ, and it's just my opinion. Next BB I want to build would look like a stock 383 Road Runner but lays down 11s through mufflers...