If I understand what you're saying, you want to put heads from a 400 sbc onto a GM 455 big block engine and you want to know if you will gain anything from it. I think all you'll gain is a lesson on incompatibility. If you're chaging heads the compression is very important and should be of some concern. It doesn't make sense to put heads from a smaller engine on a big engine. It usually wont flow as much air/fuel mix as the heads designed for it. That's probably why no one here had any experience with it.
AL