400-450hp isn't that hard to achieve, realistically its around the same limit for the same era Pontiac 400 that start cracking, so that's a non argument. Grab a bolt on halo girdle for cheap insurance, KB pistons and stock heads port matched and opened up with bigger valves, cam to match final setup. A shaker TA will use the Performer intake unless you like having dramas so that limits breathing somewhat. Headers are also an option. I have a 380hp\480tq 403 that uses the lower intake and Thornton exhausts manifolds and mild cam so left some power still on the table. I like it simply to be different and not a chev. It's a fairly standard 403 build these days, a search will bring up plenty and saves retyping.
