My 64 has a rear crossmember shaped for mounting a spare. A '73 Newport gas tank will nestle in there, but I need to add a crossmember to support one end of the tank.

Spectra Premium has a huge selection of gas tanks and you can troll the pages looking at dimensions.
If you can stand to put a hole in the side of the bed, a F150 or Bronco tank will fit back there, too.

Measure twice, cut once!

R.

Scarebird has the kit to add disk brakes to your spindles. Sometimes they sell them on the 'bay.