Well, You're going to have to do Your own detective work here.
1) As dukeboy_318 said, those are some lame-*** lo-po mills stock, and just putting a set of blue Felpro's on would likely drop the OE squeeze below 8:1. So unless that
things got better slugs or they whacked the 906's a ton, that's where the lazy lies. If they did whack the heads, look for the slightly raised portion of the intake surface to
be almost gone, assuming they cut the intake face to maintain the manifold alignment/height. Do a warm compression check, throttles wide open, see what You've got.
