There is no "gas only" tank. Stock Spectra 18-gallon rear tank is the F1D, gasser or diesel. Best price on all the Spectra tanks I've found is on Amazon, free shipping even without that "Prime" nonsense.
Spectra tanks, the number designates the location/size (So F1<whatever> is rear tank, standard 18 gallon, F6<whatever> is 19-gallon mid-ship, etc.). The letter AFTER the number designates the "hat" opening, which varies from year to year. For '85, it's D, so the stock rear tank is F1D. The oversize 38-gallon rear tanks are all F26<whatever>, so for your generation of truck an F26D would be the right choice. Search Amazon for "Spectra F26D" and you'll find one for $150, with the kit for extending the pickup and the longer bolts and all.
Our trucks (stock) have neither a frame-mounted nor an in-tank fuel pump. The fuel pump is on the engine. Sending unit resistance range on an '85 is reversed compared to the later generations -- high resistance is EMPTY, low resistance is FULL.
More to the point, with any replacement tank, your existing sending unit / pickup assembly should work, as long as the arm for the sending unit, and the dip tube for the pickup, can be extended for the height of the tank.
Edit - I'm seeing some other 38-gallon entries on Amazon, by other mfrs (Shepherd, Florence), for fewer dollahs. But no clue about their quality / reputation, and I don't see photos of the extension goodies (bolts, etc) on their listings. Spectra has a good reputation wrt fuel tanks.
Edit2 - Spectra F26D is only $133 at RockAuto, there's always a 5% discount code available. Shipping might make the difference.