yep, there was such a beast. my grandfather use to have a 1978 C10 lwb stepside he used as a work truck. i'll have to find a picture of it. it was blue and had black toolboxes. it had a 250 I6 (he refused to put anything else in it, when one 250 died, he had another one waiting for it.), 1 bbl carb, originally had a 3-on-the-tree but once his hip/knee gave out he had a Th350 swapped in. also had the wooden bed floor and the side-mounted spare tire on the left hand side of the bed...by the step. was definitely an unusual beast, but they DO exist.
my brother had a '64 long bed stepside back in the late eighties. he bought it with a buick 455 that he burnt up and then put a 350 in it. the paint job helped you not notice how long the steps really were. wish I had pics, was a pretty cool ride.
I have one of these, a 1979. Spent its initial life in the Saudi desert. Only 49K miles, but it rough shape. SBC 350 with the manual 4 speed (granny gear 1st). Has all the heavy duty options - big brakes, springs, shocks, 1.5 ton towing bumper. I have the original bed off and am working to restore it, but in the mean time I have almost completed a custom, low mount flatbed for it.
Ive got an 84 K20 longbox stepside. Its got a newer 350 with a 4 speed manual. Truck is in great shape just needs some wheels and tires. They sure are beasts but never seen another one like it. It has the wood bed and a 5th wheel that may have been aftermarket. Probably was a Texas truck at one time as it has no rust. I could see how one could pull a 5th wheel trailer and still get in and out of the bed with the steps. Ill try and round up some pictures.