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.