400 horse from a 455 is easy... All Pontiac motors are the same basic block configuration from the 301, 326, 350, 389, 400, 421, 428 & 455. Internals are different from size to size and year to year but a little judicious research will give you some insight.
Here is some info... There is a ton of this stuff on the internet. Do some searches...
http://www.carcraft.com/techarticles...ine/index.html
http://www.yearone.com/updatedsingle...ontengid1.html
http://www.krepower.com/
Mark