My search for a shorty D100 is over

i'd hazard a guess that it was because the 440 wasn't available with a 2bbl option like a lot of the ferd and chebby stuff was.

even if the fuel mileage was the same, the perception of big carb on a big motor was immediately: gas guzzler; at basically, the worst possible time.

there also exists the probability that with encroaching emissions they weren't able to meet standards and instead of flat out castrating the chonk block and turn it into the a mediocre offering they just decided to phase it out.
Maybe so, but to counter that point, there were LOTS of big slide in bed campers out at that time. They were the rage through the 1970s. Ford kinda cornered that market, too. Although the other two made Camper Specials, nobody made as many as Ford and nobody made anything that could compete towing and hauling wise. You ever heard of a Ford "Super Camper Special"? They were F350s whose rear axles were moved back 7" to increase weight capacity. No other auto maker offered such that I've ever seen. I love those trucks. Always wanted one. They look like a big bulldog. Here's a really RARE 4WD. Look how far back the rear axle is.

SUPER CAMPER SPECIAL.jpg That's a badass truck.