Help! Intake manifold won't fit!

OP, have you tried cleaning the threads in the heads with a tap & a die on the bolts? From there, get all of the bolts started & then run them down slowly to see if the interference is from the gasket thickness, of if you will have to angle mill the intake faces. It's just an idea, I thought I had the same trouble & that's what I did. Everything worked out for me, yours might be a different outcome.