I am not sure what year you have, however here is my experience with my ‘65.
I would always smell gas when it was parked. Not a strong odor, but enough to tell the smokers at work to stay away when they smoked.
I ultimately needed a new gas tank because it was slightly perforated. Not enough to see a drip, but enough to let fumes out.
However on the way to finding that out. I tried all the basic fixes first. I was surprised to learn the getting the fuel from the fuel filler hole to the tank is quite rudimentary.
There is not a lot to replace. There is a gasket / rubber seal on the outside between the filler hole and bidy sheet metal. There is then a similar rubber seal on the inside. I don’t remember there being any clamps from the filler hole to the metal bent section that takes the fuel down a couple of inches to the tank.
In between the metal curved pipe (35 degrees ?) there is a thick rubber section the allows play / imperfections in the angle to continue to the tank.
There is a clamp holding the metal curved section to the rubber section. I don’t remember (and the schematics don’t show) a clamp between the rubber section an the metal section the is the opening to the tank.
That’s it. A very basic method of getting fuel from the outside into the tank.
The design is not great if you stuff a lot of items into your trunk. The rubber section. Probably can get pushed / have pressure on it. When rubber sections attach to metal and are bent slightly to the side, then can leak.
If I were you, I would replace the two seals on either side of tgs sheet metal. I would also replace the rubber section.
since you had puddles in the trunk, and it could not be overfilled, slosh out on the outside (plus the is a rubber seal on the cap) and then come in the trunk weatherstripping, I would suspect it is one of two things. It is also unlikely that there is a hole in the metal section going into the tank
1. The rubber connector tube has perished / developed a pinprick leak. Or 2. the clamp is loose.
My money is on the rubber section leaking. Since it occurred after filling the tank full, I suspect that the fuel was up into the rubber hose section / metal bend section. Since it was up there, gravity (and suspicions one or two occurred) it drained out into the trunk. The reason you didn’t have more fuel is because once the fuel level was below the metal and rubber sections, it was only in the tank.
These are my best guesses based on my past fuel experiences.
I would start by replacing the rubber connector section (I doubt it is expensive). I would also replace the two seals on either side of the sheetmetal (even though this is improbably the cause, if rubber has perished, then this is an inexpensive thing to do, allowing you to eliminate one more thing from the suspicions).
I would also replace the clamp or at least make sure it is snug (but not over-tightened to the point that the rubber section deforms). It would not hurt to add a clamp on the other end where the rubber section attaches to the tank.
My thoughts are that if you do all these inexpensive things, you will eliminate the most likely causes. If they don’t work, you will need to investigate the tank. However since you had puddles instead of just fumes, this is highly unlikely.
Park it outside or at least leave the garage door open fir now. Gasoline fumes can suffocate you and are highly explosive. Definitely don’t use a lighter to look into the tank (Read the Darwin Awards for more information on people who did that).
Good luck on your troubleshooting.