From my experience the biggest problem with headers is how true they are.
The nature of the way they are welded together causes warping. I had one set that kept blowing gaskets until I put them on the mill and did a "fly cut" to make the surface true. After that no more gasket problems.
From that experience, I just buy the more expensive headers with flanges no less than 3/8" thick.
While I was chasing the warped headers, I tried every type of gasket known to mankind, from pure copper to asbestos. The ones that seemed to last the best, even with the warped headers, were the dark gray soft gaskets with the steel layer inbetween.
I also always replace the manifold bolts with new when swapping gaskets. The bolt threads get messed up with age and tend to screw up the threads in the heads. The bolts are cheap. I also use the cap screw style with the allen head for most of the bolts. Some must be hex head where the allen won't reach.
