The original solution to the fuel tank filler dilemma was an extended filler neck sold by Hall Pantera. I don't know if they still sell them, but it would be worth checking into. I hate to see the kids go without eating,
If you just have to have the side filler, check with the folks (vendors) who have stocks of parts culled from cars destroyed by rust or collision, such as Larry Stock (Pantera Parts Connection) in Nevada.
The final stop would be the last vendors of factory parts; The first two being Wilkinson or Jaeckel, they may have new assemblies on the shelf. The forum member named King is another possible source. He was an original Lincoln Mercury dealer that sold Panteras 1971 - 1974, and has been selling restored Panteras ever since. Hall Pantera also had a significant stock in OEM parts at one time.