This is the correct wording for the HL4 option.

HL4 24 gals.---In Cab behind seat---For use with 131" wb. Conv and 165 wb. Crew Cab Sweptline pickups only. Not available with Hinged-Back Bench Seat.

The standard tank was frame mounted in front of the rear axle. The HL4 in cab was in addition to the factory frame mounted tank.

I ordered my 77 reg cab short box 4x4 new and I could not get an auxiliary tank. I added the new tank before I drove it. The bench seat with the fold down back would work but the release levers had to be bent on a 45 degree so they would not rub through the tank. 30 seconds with a 12 inch crescent wrench fixed that.

Again if someone can prove they had a factory short bed regular cab put up the documentation. Just because it has one now does not mean it rolled out of the factory with one 36 years ago.