My 68 Hemi Coronet is very nice, but leaks about a 1/2 cup of fuel onto garage floor after a drive. It's tank was replaced by previous owner and I think is the correct tank. The 2 vent lines are aftermarket, one is connected to the inboard vent nipple at the front of the tank, while the other J-shaped short tube is connected to the outer nipple at front of tank. If the tank is only half full or more, it leaves a puddle of fuel on the ground which is embarrassing at car shows. It has the correct non-vented cap. I don't know how the 2 vent lines work, but I snaked a thin wire into the nipple with a small hook on the end to see how far into tank they go. The inboard one feels like it is only about 8 inches long, while the outboard one is long and extends way rearward to the back of the tank. I tried swapping the 2 vent tubes so the snaked trunk one was on the long outboard vent nipple, and the J-shaped vent line was on the inboard short nipple, and it leaked even worse. In 40 plus years with many Mopars, I have never had this problem. I considered buying another aftermarket tank, but am afraid it will do the same thing. Any ideas please?

Try running a vented cap.
One of the vent lines should be pretty long, and terminate above the level of the fuel tank's top; sounds like that's what you have.
IIRC, they're shown in the FSM.

Iirc, some replacement tanks had the internal lines switched and the easy fix was swapping hoses on the nipples. But you did that already.

The long internal line should connect to the J line and the short internal line goes to the long vent line.

I'd start with verification of that and making sure the lines are all clear. Remove gas cap and put air to the internal lines, you should hear air in the tank. The external lines should vent INTO the passenger frame rail. Iirc there's a couple of floorpan holes in that area and your lines might be in the wrong ones.

The vents are supposed to drip into the rail so fuel can disappate before hitting the ground.

I imagine a vented cap will move the leak to behind the license plate.

Your vent lines should look like this.

And be routed like this.

that was what i was going to suggest.
run the vent line up into the trunk, then into a circular shape like one would do a brake line coming out of the master cylinder, then into the rear frame rail.
or do as NITROUSN suggests in his picture.
the vent must be higher than the tank.
a few years back, my buddy finished up a duster for his wife and had the identical problem. he ran the vent tube up into the trunk as suggested, then back down into the frame rail, and the problem was solved.