I had to replace my tank and back in the day they did not make a replacement for my car. So, I cut the trunk and laid in a 65-68 Mustang tank--love the flatness of the trunk and the fact that you can't see the tank when looking at the car from the rear. That said, I get gas smell in my attached garage and on occasion, when the tank is full or near full, I will have gas spillage from behind the cap when I come out to the car. I think I know the answer but wanted to hear how and what you might do to deal with this issue. Thanking you in advance for your input! The fill tube is the original (metal) as it is connected to the trunk body and goes past the metal supports in the trunk and then it has a rubber hose connection from there to the tube on top of the gas tank. The gas cap vent is stock and operational-I can blow air through it for example. Any and All suggestions on how to fix this problem are welcome.
Get a new tank for a '71-'73, along with filler neck seal and top vent seal. Then close the hole in your trunk floor with some sheet metal...
I have a 22 gallon '70 Mustang tank in my '70 Maverick. Same deal, it drops in from the top and I have a vented cap, although my cap is aftermarket. I don't have any of those issues with mine. I would see if just replacing the cap helps.
I have a later Mustang tank under floor in my Comet & don't have this problem. I take it your tank has no vent? Mine does & I'm 99% cap is not vented, no leak. Without tank vent, expanding gas will sometimes dribble out. When vent line plugged, I had this problem on my Cobra, a vented cap made it worse.