3/8 Stainless fuel line .028 wall should do just fine,
If you have the tank down you might as well put a return line access fitting in case down the road you think about an EFI.
Recirculating when using an electric pump makes a lot of sense with today's fuel, it cured my Vapor lock problem. I used a high pressure pump with a bypass regulator on my firewall close to the carb. I ran the fuel line up the passenger side frame rail and came up the firewall and set my regulator to 6 lbs. I turned my dual feed line around and fed my carb from the backside this way you avoid all that heat from radiator and exhaust......JMO
