67 Barracuda broken clips for header panel bolts?

I would use a T-nut used for wood. Just tap it in with a hammer and lap weld it. Very easy and probably the preferred method.

Or for a more professional look, make the hole just slightly larger than the T-nut diameter. Put a bolt in the nut to hold it, and tack weld it. After you tack it, check the fit with the bolts. Then finish the butt weld all around with either steel or bronze, and grind smooth.

Either way, nobody will see it, and you will not need to get inside the fender. The cadmium plating will burn off, but who cares.