Beware of tank repairs. I wouldn't go near an old tank with a welding torch. Apart from the risk of vapour going bang and singeing your eyebrows or much worse, trying to reseal and refit an old tank is a pain.
The problem, especially if the vehicle has been in a rust-prone area and/or has done a lot of off-roading, is that grit, mud and crud builds up in the gap between the tank itself and whatever protective tank-shield you have underneath it. That dirt and grit never dries out properly so rots out the floor of the tank. The leak may start as a few drips, but the whole metal base of the tank, or parts of it, may be lace-thin. Sloshing sealant around inside may help cure the leak.... only for a while.
Ask me how I know.... my 1993 XJ's gas-tank went that way (and it had lived for its first 15 years in the desert). Mine had that flexible shield strapped underneath it - it's not steel but some composite stuff. I bit the bullet and fitted a new tank.
So, if you do take the fuel tank out, test it vigorously - perhaps with a screwdriver, especially across the whole of the base and along the seams. Rust attacks everywhere.
Obvious other tip: when you re-fit or replace the tank, first give the load-space floor above it a good check too, and rustproof that panel and the outside of the whole tank itself while you have decent access.
