Before you do anything more, if it is possible, take a small hammer and tap along the frame sides, especially in the area around the "kick-up" over the rear axle. If you feel/hear a solid "clunk" that would indicate the frame is solid. If it feels spongy when you tap it, or sounds like hitting plastic, or you punch a hole in the frame, I would do as Chev Nut suggests and RUN AWAY! My thinking is that if the body mounts are badly rotted, the frame is probably rotting from the inside out as well, so the car is not worth spending a lot of money to rebuild.