Just keep an eye out, there was one here at the local Ford Dealer, they had used it for parts deliverly, but it sat out back until the owner retired and I do not know where it went. There is also one sitting out in a flield on my way home with some other junk. So keep looking. I did have a friend that put in a 390 with a C-6 trans in a Econoline van, mid engine with a short drive line, if you don't mind a engine in the bed this would work. A manual would take some thinking, a hyd. clutch would work, but the shifter linkage would be the problem.
I'm sorry I don't know much about these trucks, but I have seen them with 5 windows and with 3 windows. Good luck on your search.