I looked at doing this. Should be very straight forward. I was going to do this to the crewcab I'm working on, but got to thinking I just need to get the darn thing drivable. I love the Crewcab extended cab trucks. 6 doors seem way too long especially with a normal size bed.
Body lines match up, rear doors have the same profile top to bottom as the front doors. Rear plastics from an extended cab would work inside still. Seems like a really easy swap. I was concerned about the frame kick up behind the crewcab so I think the rear floor of the exteneded cab may need some massaging to get it done. I would try to round up a 92 or newer extended cab as they have rear shoulder belts and the better looking side windows. Carpet would need to be seemed probably under the rear seat and the headliners would get seamed also. That maybe able to be done as one shot so it doesn't have a visible seam, but it also may need to have a visible seam.
Price to me would also depend a lot on if a guy has to repaint the entire truck and do a lot of rust repair or if a guy can just blend the added extended cab into the rest of the pickup. Flat bed would keep it simple, but modifying a stock bed to fit wouldn't be too bad. Worst part about modifying an older truck like these is the repair work that needs to be since they are older.
I'd love to take on a project like this for someone if I had time. I don't have money to fund all the awesome projects I'd love to do. Definitely don't have time until next spring or summer though. Gotta get the crewcab done, oh and the 93 running again.