Fuel sending unit.

I put the 3/8" one w/ 1/4" vent in my 64 & 64 A-body. I use the vent for an EFI pump return. Otherwise, 5/16" is plenty and what my 65 Newport w/ 383 used. 90's Dodge pickups w/ Magnum V-8 also used 5/16" supply as I recall. You will find more filters and fittings in 5/16" than 3/8".