Oh, I'm not saying that it will not remain level, but you can definitely tell the difference between the chassis being flat on the ground and jacked up by one corner. Mustang chassis is pretty weak, when you compare it to other alternatives that are used in road racing. It requires a lot of work, some of which is too much for a street driven vehicle. Making a cage in the street car is simply unsafe with out helmet, seam welding costs most than the whole car. There is just no two ways about it: you either have to build a full competition car, or do as much as you can with out making it unsafe and live with the fact that chassis is not as good as it can be.
Also, my subframes were installed on a drive on lift, by a guy who builds custom race chassis and cages for living. Pretty sure everything was welded on correctly.