I caved in one of my rear jack points by using a normal hydraulic jack. I bought the jack adapter from Flyin Miata that night. The only thing is the warped rust can hold the adapter in place on the car when you lower it, making it easy to forget it and lose it somewhere on the road (almost did that)
Hey, at least you have a jack point to cave in. Mine is a lot worse than I thought. The pinch weld along both sides is real bad and the rear jack points are totally gone. I pulled the car out of the garage for the summer and went poking around. Needless to say I'm about ready to dump it. If only I could have bought this car before the previous owner let it rot.