I found this when I was looking at prices for a roll cage, Does anyone know how this works? I can't seem to get where the body comes in http://www.jegs.com/i/Jegster/550/942405K1/10002/-1
You would just hack out the existing chassis for the most part with that chassis kit, and then drop the body down over it. Then form new floor pans out of sheet metal. They sell wheel tub kits that could be trimmed to fit. The whole thing would still be a lot of fabricating. This chassis kit gets you halfway there.
S&W race cars also makes cages for our cars that have actually been used in cars before. And out of the thousands of cars on here theres maybe five that requires that type of cage.
Usually, there is extra tube in the kit. The way I have seen it done is you level the chassis at ride height, set the body over the chassis where you want it (centered front to back and side to side, at desired body height, etc.), measure from the bottom tube of the chassis to the rocker panel, cut lengths of tube to fit, and weld the body to the chassis that way. I've also seen where people have removeable body mounts, but I haven't actually seen how they did it. Hope this helps, -Scott H.