I built a 8.5:1 406 with Comp 268H cam, Performer intake and Hooker headers. It was less than spectacular. I later took the heads off and had 2.02/1.60 valves installed and it made a huge difference. Power was up where I expected it to be. IMO, don't bother with small valve heads on a 400. You would be leaving a lot of hp on the table.