A. B.,
On the early 1980 FJ45 longbeds and newer, the fuel tank is under the cab, just as on the January 1979 and newer FJ40s--same tank, gasoline or diesel. I strongly suspect that when the factory made the detail changeovers/improvements for the 40s in 1979, they revamped the entire 40,42,43,45, etc. lineup at the same time, both gassers and diesels. That date would be all of 1979, and it was a major milestone. August 1980 was another changeover marker for the lineup, and October 1982 another. There were some Japan-only(JDM) specific details throughout the years, and likely a few trucks got caught during the changeover month using what ever sat in the parts bins on the assembly line.
Your passenger seat arrangement should indicate your actual tank placement: clues: no floor-depression under the seat = the fuel tank would have been placed under the cab, slung mostly under the passenger's area, all the way to the very rear back edge of the cab, supported by 2 straps anchored fore and aft. There would also be an exit hole for the filler behind the seat on the raised floor near the outboard wall of the cab. A circular 5-inch cover plate sits mid-point behind the seats for tank sender access.
If your tank is missing or has been replaced over the years by something different, I believe a gasser tank from a 1/79 up FJ40 should fit.
My two cents.
