Danger:
In theory, anyone with a floor jack, jack stands + car = a very crude light field press.

STOP,Bad idea...
Using the jack to press the part in, by lifting against the car is not a safe procedure.

What can go wrong:
* Car falls on you (loose a limb or die).
* Work piece or jack shift (injury).
* Dropping the car (oops) vehicle/equipment damage.
* The irregular working shape can Bend the sub-frame or punch through the body, due to application point pressure.