I'm replacing my fuel tank soon because of a leak past half way mark, looking at sending units and figuring out which one I need. My car is 350 with a.c., which is deleted though. My fuel pump has 2 lines (one to tank and one that's blocked off). Im trying to figure out if I should get a return line sending unit and plumb a new return line?. Or just get a sending unit without return line?. Planning on putting a mild 455 in next year also.

 

There's no reason not to run a return line (other than the hassle of installing the line on the frame rail). Fuel return helps mitigate vapor lock and percolation.