If I remember correctly, the tanks are constructed with polyester resin, using antimony trioxide as the fire retardant (which makes it difficult to get a patch to adhere). You could probably do the repair on the interior of the tank (with considerable difficulty), by working through the holes for the fuel level sending unit and the gas cap. My guess is that 3 layers of 6-Ounce e or s glass extending about two or three inches either side of the seeping seam at the leak location would probably do the trick. However, that is just a personal opinion, not a recommendation. You would need to talk to an expert to get an actual recommendation on how to fix it.
Good luck,
JimC
