Just what is the purpose of the return line? There is a return line on my 67 special AC car, No return line on my non AC skylark... motor in sig below... On my lift pump it has a second port on it... FWIW the pump has the larger cup on it.. Cup = I have no idea what its called.... kind of looks like this -https://www.autozone.com/external-engine/fuel-pump/p/trugrade-fuel-pump-b1263mp/562495_0_0
Return lines in that era was "hit or miss". The driving factor was vapor lock, so in some cases the "mindset" was an AC car would be in a hotter environment, and the extra load on an engine with the accompanied heat from evaporator coil, compressor load etc., could be enough to create a hotter under hood condition creating a vapor lock, contrasted by a non-AC car. Cars in the late 60s and early 70s, with the EPA mandates, fuel quality/blends, also contributed to higher under hood, as well as engine operating temps, and issues with vapor lock. You almost cannot have a negative impact with a return line. If you can run a return, it will not hurt.
In '70 on a 350 A-body, only A/C cars got a return line (A/C included HD cooling but HD cooling itself didn't get a return line). In '71, HD cooling got a return line regardless of whether the car got A/C or not. So to your point, it was hit or miss and I guess in '71 they figured if someone was ordering HD cooling on a non-A/C car, then good chanve it was going to be used in a hot environment & the return line could be useful for preventing vapor lock.
The return line circulates hot fuel and vapor back to the tank. The idea was to keep cooler fuel in the pump, and it's purpose was to prevent or lessen vapor lock. Bob, A description of the vapor return is in your 1967 Buick Chassis Manual, page 64-5, section 64-4, paragraph b. https://www.teambuick.com/reference/library/67_chassis/files/64-a.php