If no one will sell you a '73-up left side gill assembly at a reasonable price and you're handy in the garage, it's possible to build your own of molded fiberglas and hole-saw a 3-1/2" opening in the new gill. Then the cut piece becomes the swinging cap. I did this 20 yrs ago for a custom right-side opening 'gill' to have radiator cap access for an altered swirl tank. You could also saw a hole in your rather expensive stock die-cast gill. For the hinge, it can be made of scrap metal; you'll likely need to borrow a late left gill to reproduce the necessary hinge parts. But that's only the external part of this conversion, I'm afraid.
You'll also need the later gas tank spout & joining hose, and after sawing a big hole in the body, add the gas cap extension tube just behind the new swinging door. There's a late left-side top engine cover which has a swelling around the new style tank spout. A less satisfying alternative is to chop a clearance hole in your early right engine cover around the new tank spout, which will always be visible afterward. Flat-black paint hides a lot of ****....
All these parts bolt onto early Panteras, but the once-offered 'kit' was never cheap. The parts necessary for the conversion are shown in the black or orange Parts Manuals and one or two vendors may have all the pieces. Is there a Pantera club/chapter in your area, or do you have a good friend with a late model, non-running car?