The sundancer simply added a section of subframe to bridge the unsupported floor between the front and rear subframes. The piece they added was about equivalent to the frame rails on a stock CJ7. It doesn't have to be very big or thick.
I personally think its a terrible idea because you don't have a tailgate and because you are starting with a four door. I want to create my own convertible one day, but I will start with a two door sedan. The seats are behind the post on the four door, so you would have to make a very large targa bar or a second bar. I don't see how it can be done without being hideous. I also just don't see the point of a pickup bed without a tailgate.
There is potential if you make a softop over a full length roll cage. I'm imagining a softtop station wagon, and that would be all sorts of awesome.
I'm enthusiastic about making one of these out of a Kammback. I'm sure that will be a knife in the side of the purists, but the kammback I'm starting with has no front subframe left. Most people would have scrapped it already. I'll be making a new subframe and continuing it the length of the car. The original lights and plate in the rear will be modified to hinge downward. An XJ fuel tank is the exact same size as the smaller SX4 or Kammback fuel tank, except that it has the fuel inlet on the drivers side instead of the center. My kammback will have an XJ fuel inlet on the side and a XJ fuel tank. That tank doesn't fit into a four door sedan without modifiying your spare tire recess. I will continue the bottom of the window rearward and remove the roof behind the pillar while keeping the slope of the rear tailight area. Here's a picture of my other Kammback to imagine that with.
I suggest making one out of a two door Cherokee. I love those when they're done right. The Commanche tailgate really ties it together and makes it practical as well.