It depends on the fuel pump used. If the pump was designed to use a return line it's probably a good idea to hook it up. You can go either way, replace the pump or replace the matching sender. I would do the pump. Easier to get to and it sounds like your sender is new.
The 400s had the return, and the a/c cars I believe because they had the bigger volume pumps. My 350 (non a/c car) didn't have one. Now that I'm running a 400 and a larger carb the stock 350 pump couldn't keep up.
