The factory used 5/16" up to low po 440's and 3/8" for high po 440's and Hemi's so 3/8" will run a lot of power. It does depend somewhat on setup - for example mechanical or electrical pump.

I've gotten sending units from Mancini - 3/8" with 1/4" return. Just about all of them need the sender tweaked to get a somewhat accurate fuel guage reading.