return line for EFI conversion

Lookup my posts on this. 5/16" will work fine, but since you have a choice, use 3/8" for supply. All the new senders I have seen have a 1/4" emissions line. I used that for return in my 65 Dart, but measure 9 psi return pressure in the engine bay with my Walbro fuel pump (225 lph I recall). Next time I pull the sender I'll try flaring the 1/4" tube for less restriction.

In my 65 Newport, I drilled a hole for 5/16" return in the sender and rigged up pipe fittings w/ an O-ring seal. Never had a leak there. I measure <1 psi return pressure using a Holley Pro-jection 2D with their gear-rotor pump. Holley specs <4 psi return as I recall. I doubt the world ends if higher, as long as the pressure at the injectors is regulated correct. I run 22 psi there.

BTW, I put my EFI pumps on the engine bay frame rail and never had a problem. Maybe I'll starve the pump if I climb up a really steep hill in San Francisco, never tried.