I really should not jump in here, since my experience is not directly relatable, but it may give you some ideas.
I had a swb F150 I bought new in '79. Damn tough truck. C6, with stock 205.
I added the big bumper and the PTO winch.
Then, in '89 I wanted a new truck. Trouble was, the car makers were still recovering from the gas crisis, plus new gov. rules.
The heaviest duty 150 (1/2 ton) Ford, was a wimp compared to my old one. So, I decided to go to a 250 (3/4 ton).
But, they did not make it in a swb.
I cannot use a lwb in the woods. I need to be able to maneuver.
So, I special ordered a 250 with all the options I wanted.
Superduty cooling, snowplow option, ( I bet that one blew their minds since it was going to Fl.<G>. But, it got me the extra heavy duty front suspension).
When it came in, it went straight to a local truck customizer.
They took the lwb body off, and cut the frame in front of the back springs.
They had a swb truck sitting there with no rear body, and they just took out enough to make mine the same distance from the front to the back.
They took the rear driveshaft to a machine shop friend of mine and he shortened it to swb specs and balanced it.
The exhaust system already had an extension in it, they took it out and the ends went right back together.
I think there was a splice in the brake line, and they just rerouted the e-brake cable to allow for the slack.
The front gas tank would not work, so they swapped it for a swb front tank. Rear stayed the same.
Lastly, they put on a second hand swb rear tub. All the bolt holes lined right up, and everything fit.
In other words, the 89 Ford pickup used all the same things for both swb and lwb. They just added a section in the frame, and a longer rear shaft.
Which makes sense, that is the cheapest way to do it.
Of course, later I swapped out the BW 1365 transfer case for a NP205, added the hydraulics, massive front bumper, winch, and other stuff.
But I would not be surprised if GM did not do the same thing.
