I have a NOS 26 gallon Blazer diesel aux fuel tank in the spare tire area on my '85 Toy 4runner. I got the Blazer tank from Coleman's Surplus of PA, but they haven't shown it in their catalog or web site for some time. I do, however have the GM part number if anyone is interested.
With the 17 gallon OEM tank and one five gallon jerry can, this means I can carry about 45 gallons of fuel while conducting long range desert exploration trips.
In the enclosed pics, you can see that the tank does not extend below the rear spring hangers at all and it is well tucked-up under the rear of my 4runner.
I welded a 3/8" nipple on the lower right (passenger side) of the tank, then connected this to the OEM fuel tank via a 3/8" ball valve, an inline filter and an OEM tank fitting from NW Metal Products. I gravity feed the fuel to the main tank while parked, not while driving.
Good luck.
Al