The Spectra Premium F26D and F26E are the rear 38 gallon tanks, the spare fits under them.
The Spectra Premium F8D is the 32 gallon Bronco tank with large fuel sender hole (for the OBS fuel sender), its deeper so the spare cannot be mounted under it.
Spectra also makes a 16 and 19 gallon side tank, depending on wheelbase, go to the RockAuto.com website and determine which one you need. Starting at about $70.
The Spectras seem to be factory quality and are the cheapest!!!.