Just hunt down an auxiliary tank. 80s, outside of NA, have them. Heck, Australia's junk yards are littered with them. They have to move these and center console fridges out of the way to get to the spare ambulance doors.
You'll have to buy the dual fill neck, pick up, pump, switch, brackets, gauge and sender too. There might be one other part I am missing. There are several threads about adding a 13 to 14 gallon OEM aux tank on here.
If you are frugal like me and you can't drop that much cash, I have been eyeing the late 70s/ early 80s Chevy Blazer or Ford Bronco fuel tank. They measure 28 x 28 x 11 and should fit there nicely without too much hanging down. Eventually I'll be getting one and trying it out.
Wasn't there someone who used a 4Runner tank in that space? I'm on the iPad right now, and the searches don't always link to the right threads for some reason, can anyone on a real computer help a brother out with a link?
I'm stripping my 80 for the moment (replacing chassis, it was badly ripped at the steering box )
I have used a main tank from a 70 series as subtank. Fits nicely where the spare is supposed to go.
If I remount everything, I will take some pictures of the setup while installing it.