A 1/2" (or -8AN) return line to a RobbMc 1/2" sending unit will work fine. When using a bypass/return style regulator, ALL the unused fuel is returned to the tank (unlike a bleed or "vapor return" line which only sends a small amount back to the tank to keep things cooler). So when using a bypass/return regulator, the regulator won't work very well unless the return line is large enough to eliminate most of the resistance in the return line. With a 140 gph pump, you will need a 1/2" or larger return line to make the regulator work correctly.