Replacing fuel tank on 1966 Barracuda, which sending unit?

I am ordering a new fuel tank for my project, the orignal one is dented and has a fiberglass or something patch that isn't holding up. Want to replace it with new and move on to the next step. I am ordering the spectra tank from Rockauto.com but not sure which fuel sending unit to purchase. There are two listed, one has 5/16" Outlet and one has 3/8" Outlet; w/ Fuel Return..which one would be the correct one?