It is not untypical to see warping that results from welding. It is welded-construction, not cast, so it barely matters, as it will likely yield to flat upon torquing the nuts on the studs. Local warping could be more problematic, like if the curve is welded in place by the header pipes - use a shorter straight-edge to see it. What is more important is equal thicknesses of flanges, header and intake manifold. Measure both with a micrometer or calipers. An exhaust leak at the head is dangerous, so depending on how it is warped, you might change your approach to gasket material, and gasket-maker vs no glue. One header flange that I had was thicker than the intake, so sealant on the intake manifold is going to be essential, Remflex gasket, using a custom stepped-washer, etc., it kinda depends on the geometry that you are trying to correct.
Without any further research, I'd trust the header and assemble. I like anti-seize and a torque wrench, thread file or equivalent for prepping the head studs.
