[oldiron]

The return line is used on fuel pumps with three fittings. I believe that in 1971 these pumps were used for emission controls. If you don't have to have the truck checked for smog controls I would leave the return line off and stay with the single fitting fuel pump.
Since I only have experience with 3/4 tons they were not required in 1970 on C20's.
Also use a vented fuel cap when using just the feed line only or you will have a collapsed fuel tank with a non-vented cap.
I noticed that you do have the return fitting on the fuel tank. Your choice of either capping off the return fitting on the tank or install the fuel pump with the return fittin on it.