The side/front tanks in our trucks extend a little under the back of the bed. Can a person put a short box tank easily under a long box and have it not extend under the cab?

 

what goal are you trying to achieve here? your idea should be possible, or you could be able to just run the rear tank if that would suit your needs, maybe the high volume tank found in the bronco

 

Making a truck bed trailer, figured might as well use the tanks to carry extra fuel. But of course I can't have anything but tongue protruding forward of the bed.

 

ok, the rear tank position is wide open for that use, and since you don't have to worry about a driveline or exhaust, you shouldn't have much trouble mounting another rear-style tank forward of the axle.
it wouldn't hurt anything to carry your tape measure around the junkyard and see what dimensions are offered there too

 

Yes.The short bed tank is only 14 gallon so it is that much shorter than the long bed tank that is 19gallon