Before any cutting of sheetmetal or framerails can begin, you must make sure the body is securely fastened to a supporting system and perfectly level. You must ensure the remaining body retains its exact original shape. At MCR, a frame jig is available, but we know you probably won't have access to that kind of equipment. What you can do is make stands for square stock, much like the units that are part of the jig. If you can get a set of jackstands to perfectly level the car, that'll work too. Just put a small tack weld on the stand where it meets the car to ensure it doesn't move. Also mark the stand's location on the floor; that way if the stand gets moved somehow, you can put it back.