is there a way to plug the fuel lines

The fuel lines from both tanks run into the same fuel line so with the primary fiuel tank off, the fuel lines that go to that tank are just open and the return line let's out fuel when its running. We are trying to plug those fuel pump lines because that tank was rust damaged and only want to use one fuel tank

Yes I'm pretty sure I have never looked for it but pretty sure the electric switch works because its set to rear tank and rear fuel pump is operating its just that fuel still comes out of return hookup that goes to the fuel tank that we removed. We cut a male fitting off old fuel filter and going yo try smashing and welding shut the cut end and just clip it into the return line connector if they don't make those plugs .

So the switch valve that switches tanks isn't working correctly? If its changed or repaired there shouldn't be fuel coming out of the return line that is left open

Tell ya what I have a 1990 f150 was having the same exact problem except pump and or sending unit was bad in front tank well it ran fine on just the rear after while and as of about 3 hours ago I took front tank off and plugged plastic lines with brass plugs with simple 1/4 inch hose clamps and also put screw in breather tube as well and I'm currently running off just rear. If this doesn't pan out I will be running new fuel lines from engine to rear tank but was looking for the cheapest solution at the moment. I will post any updates as they come up. Apparently there's recalls on half the F Series trucks fuel systems between 88 and 96 and there never been a real solution aside from replacing both tanks pump assemblies straps and all under warranty and recall which most have expired by now.

Also the check valve is built into some of the pumps on these trucks as opposed to being in line and bolted to the frame rail.

1990 through 1993 they had a recall for something called a fuel crossover issue cross flow issue Google that