EFI tank with Carbureted engine plumbing questions

The Holley 12-880 regulator will work for you. You can use the existing 5/16 line for the return and use the new 3/8 line for the pressure side. You don't even need 3/8 line on the pressure side, 5/16 will work fine there too. When you have 60 psi in the fuel line the line doesn't need to be very large.