I was going to respond to this with essentially what
Marc described. I have two sets of SW Headers and to get that flange straighter/flatter than what is described by Dmod81 or perfectly straight would be almost impossible without planning the flange surface on a milling machine after it is welded up. The Flanges are relatively easy to press to the Head exhaust surface with the Header Bolts and I am sure after a few runs get HOT enough to reduce the internal bending stresses caused by the Press Fit (Bolting).
I would not describe the issue as Warped SW Headers but rather a normal tolerance associated with welding on a jig table which is of no significance when bolting the rather flexible flanges to the inflexible Head Exhaust Surface.
This is a situation of Close is Good Enough and .045 inches is pretty good 