Ideas on running a fuel return line

I am doing the same, but for a high-pressure fuel pump feeding a TBI. The fuel sender I recently got has a 3/8" outlet and 1/4" return. I am using the factory 5/16" tube for the return. It sounds like your sender is different since your small nipple was for a charcoal canister. Surprising for 1972, since I thought those started in the late '70's. Perhaps yours was a CA car.

Re vapor lock, interestingly newer engines (~2000+) don't have a return line. Pump bypass is done in the tank, with only a single tube to the engine. I understand that is an EPA reg to keep the gas in the tank cooler & limit boil-off. The higher pressures for MPFI (>50 psi) must prevent vapor generation in the tubing.