Seems to me the first thing to start with is the frame. Figure out how much you want to stretch it. Cut the frame and add new frame to it. Make sure that your lenghts are correct before welding. Probably wouldn`t hurt to get measurements before cutting.Make sure you "sleeve" the frame to giveit strength so it won`t sag.Of course you`ll have to lengthen the drive shaft, brake lines ,fuel line.After that you`ll probably need a doner car.If you want to make it a 6 door I would get the frt doors and cut both the hinge pillar and latch pillar out cut a section out of the floor to fit the gap making sure it overlaps by 6 inches on both sides, cut the middle of the roof out cause it`ll need overlap also the curves at the frt and back won`t lay flat.If your going to have a panel in between the doors just get some sheet metal and weld it between the posts.Make sure you use all the inner structure as well this will give your body strength.Well sounds like alot of work Huh? But it can be done,if you set your mind to it.Hope this helped.