The sealant I use is Bondo! Clean the crack/pinhole with alcohol and let it dry for an hour. Let the Bondo cure at least 12 hrs; longer is better. My mid-tank pinhole was bondo-ed in the last century & still holds. Some day I need to braze it. Not sure about the chemicals in other sealants but plain Bondo IS fuel proof once cured on clean steel.
You probably know this, but the fuel tank comes out only after pulling both the engine and ZF. Once the powertrain is out, you also have access to the mid-car welded crossmember that passes under the oil pan. Cut it out and have someone weld end-plates on. The now-removable crossmember is held in by the lower a-arm studs so no extra bolts needed. The exact length needed varies for every car.
Until about 1975, removing the oil pan also requires the engine to be pulled. The small bracket holding the e-brake linkage also needs to be cut and made removable with angle-iron, all so you can remove the oil pan for any reason. This factory upgrade & parts are listed in DeTomaso's 1972 Gr-3 catalogue and in the post-'80s parts list. Ford chose not to offer it when they controlled the project. A large number of early cars already have this necessary mod.