Bumping an old thread. Don't see why this wouldn't be super easy actually. Today we saw the aftermarket potential for new fenders on the SEMA thread. Why couldn't there be a simple aftermarket kit to do what you want? It would be 2 main parts;
a new fender part that connects all the way forward at the rear door jam
a new hard top/soft top piece that includes a full length window. I don't really see it needing anything else really? Maybe a latch hidden somewhere inside so you can remove the whole piece of fender using the already there door latch.
The front door would still be the original shorter kind from the 4 door. This keeps it simple I wouldn't want to cut up the original door jam.
A kit like this for $4k, plus rear seat removal kit from a reputable company would make quite a sweet rig. I'll admit, your version looks a bit better with the longer door, but I'm not sure how realistically we could get that compared to mine.