Check the impression on the gasket when the headers are bolted to the head. I have a set of headers that had to be trued.
What I mean is the header didn't leave an imprint all around on the gasket. I spent probably 3 hours trying to true up the headers, alot of filing & grinding.
Most headers will look straight when they're not bolted, but when bolted they will flex as you tighten the bolts. Usually the outer edges will make contact but the upper and lower portions will not.