On the Hall headers, you need to flatten the #5 pipe. It won't clear the gas tank heat shield otherwise.
Those headers weren't primarily built for the Pantera. Gary found a "boat" application for them and they were close enough.
As suggested, you may need to use studs so that as you tighten the nuts it will pull them into a better alignment but don't be surprised if you wind up having to heat them red hot, tube by tube to make them fit.
Incidentally, that heat rap has a reputation for causing the headers to rot out at an accelerated rate.
As silly as they are, the original headers (which rot out under the heat shield) and the "GTS" headers fit pretty well "out of the box".