I have found I can use short pieces of 3/8 and 5/16 fuel line and slip the nylon line up into the rubber fuel line and use a hose clamp. Always worked for me. Just don't tighten the clamp down too tight or it can collapse the plastic nylon line. The supply lines are 3/8, the return lines are 5/16.
So at the rear sending unit, I have a short piece of 3/8 and a short piece of 5/16 rubber fuel line, slipped over the metal sending unit lines with hose clamps, and then I am using the original nylon line slipped up into the rubber lines with hose clamps on them.
So at the rear sending unit, I have a short piece of 3/8 and a short piece of 5/16 rubber fuel line, slipped over the metal sending unit lines with hose clamps, and then I am using the original nylon line slipped up into the rubber lines with hose clamps on them.