I thought about building one once, by cutting a 1st gen cab right behind the doors, and another cab 1/3 of the way along the door area, splicing the two cabs together, building the front edge of the rear door frames, then modding a couple doors to be straight on the front.
It's a little more complicated on the doors, because the top edge of the door (not the window part) is shorter on the front. So you'd have to slice back horizontally from the leading edge just under the window lip, pull that piece up, then scab and fill to get it level with the rear part of the front door window sill.
The upper window track could be cut off two more donor doors, from the middle top back, and then flip the two pieces from one side to the other and reverse them, creating a squared-off window track on the front of each rear door, instead of the sloped way it is stock. You'd have to have custom glass cut or use Plexiglas for the windows.
If you could find a set of rear crewcab doors from Mexico? Canada? Australia?, shipping would suck but it would be easier.
I have FlexiSign 4.5, GOOD sign-making software, and have put a good side-profile pic of a crewcab into it and used the measuring tool to measure everything. I tend to build rather than buy, when buying would be painful.
My donor 1st gen turned out to be FAR rustier than i thought, as i found out one night when the guy who had the shop next to mine had a party for some friends from out of state, and coerced me into setting off some explosives to entertain them. I ended up scrapping the leftovers.