Jason at BGM Metalworks did a nice job moving the tank intake tube rearwards (even re-painted the weld area), and now the tank fits as it should. The setup requires a new breather tube, as the old one is now too short, but that's not a big problem.
Tubes now fit, but the forward-extending of the holes in the tub are still necessary.
Tank re-installed, with filler tube attached (Land Rover has a plastic tarp cover on, to protect during painting other parts). Breather tube will go on once it has ben obtained. Triangular brackets sitting on the tank are the lower seatbelt mounts, which will be test fitted and be used to drill the new holes they require in the sills.
Seat belt brackets are just waiting for the seatbox paint to dry for a day or two.