Wrap a piece of chain under the jack, loop it over the frame and hook it into itself. That puts all the pressure between the jack and the frame, and gets the vehicle weight out of the picture. I've straightened frames that way.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!