How to block off fuel return line in Classic Industries Fuel Sending Unit.

I don't even like those caps for vacuum. Years ago I had one blow off with a backfire on a cold morning, and one other had "gone soft" and the fitting sucked a disc the size of the tube into the engine. Did not seem to hurt the engine but the 3/8 fitting caused a ridiculous air leak

I know it's ugly, but a short hose with a sawed off bolt and a pair of clamps WORKS.

Or take it apart and solder it shut. You might "trial fit" and see if some bolt size will accidently "almost fit" perhaps you can tap the inside of the fitting, "dope up" the bolt with some good sealer, and screw it in there.