You didn't exactly say where the leak occurs but a good guess is at the vent location by the license plate. Most old Fords have a somewhat similar vent system but with the vent end on the left side (driver's). Your rear location and lack of height to the vent line is, with gas sloshing back on acceleration, likely the source. I'd try running a rubber line to the vent on the tank (to the left of the tank) raise the center of that line up a least 4"-5" above the top of the tank and then route the open end down thru the OEM Ford hole in the fender well (or drill a new one). Can rig a temporary setup with some zip-ties to experiment.
Had a similar issue with my California-built '70 Stang that had a weird and new at the time special gas tank & fitting on the tank's top that vented fumes to a small charcoal filter. Bypassed it with a similar setup mentioned above.