[bnoon]

My two carbed trucks had return lines on the frame. They were both 3/8" for the feed and 5/16 on the return. Note that the return line was down by the mechanical fuel pump FYI and not up near the carb feed line.

The only problem with running one pump inline for fuel injected is if you ever run out of fuel, you may have issues priming fuel to the pump. Personally though, I've never had any issues getting them to suck fuel if you keep them within about 18 inches of the tank, which could be difficult with dual tanks.