Fuel coming out of return line ?????

My 68 GTX has the 5/16 fuel line and 1/4 return line. I don't use the seperator just an inline fuel filter. The return line is not used. Car has the 2 vent tubes installed look to be orignal. 3 years car has been fine. Yesterday went for a short ride stopped to get a coffee and saw fuel pooring out under the car. Opened the hood and saw fuel coming from the return line. Never did this before. What could be the problem? I plugged it off with a vac plug to stop the fuel for now. Just wondering why it was coming out of return line.

Mainly because you don't have it hooked up right, would be my guess.

Put back like it was new and you won't have problems.

Explain? I didn't hook anything up. Car has been fine for last 3 years i've owned it. Tank and lines are orignal. Only thing changed was not useing the return line. Fuel line to pump to carb just like a 383 car. No vapor seperator so no 1/4 inch return used. Fuel is coming from tank out the end of return line under the hood. Wondering if vent tube is clogged?

Not saying that is the cause but yes I'd check the vent tubes. I had a black rubber? cap on the return nipple on my bypass type filter & it sprung a leak after several months and I've had a red plastic type vac nipple cap on the return nipple port on the sending unit & no leaks after 5+ years

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Wondering if vent tube is clogged?

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That would be your best bet, the factory vent lines (2) can be clogged up over the years for a variety of reasons. Dirt, undercoating, crushed tube, rubber deterioration, mud dauber wasps, etc. Your gas cap is non-vented otherwise it would leak gas every time you accelerated. With the gas cap holding pressure, the vent tubes are supposed to do that job. If they get glogged then the pressure build up has to go somewhere - out the return line. I would get comfortable underneath the front of the gas tank and trace your vent lines to see what the condition is. One line is pretty short, maybe 10", and the other goes up into the trunk, then over to the side and back down into the axle tunnel on the passenger side. In the picture, the vent lines are the two small ones over on the driver's side of the tank. Open the gas cap and give the vents a short burst of air. You should hear it inside the tank. Once those are open just plug off the existing return line. I had a mud dauber wasp close mine up years ago...