I had a similar problem, but on a totally different vehicle, for what it is worth.

Thorley headers on a Chevy 350 small block. One sealed just fine, the other did not at all. The flange was warped on that side, I put a straight edge on it and you could see the curvature of the flange. The bolts would torque out on the wrench (click) before the gasket was properly preloaded, resulting in a blown gasket. You could not see a gap, but there was not enough pressure on the face of the gasket.

Thorley ended up replacing it, but it was a pain, had to send them the header at my cost to evaluate, then they reimbursed me.

I would get a good, known straight edge and check the head and the header to see if either is out of flat by a significant amount.