do the subframe bushings have hollow spaces in them? If they do find some two part urethane...mix it up and pour it into them....let it cure and then install these spacers....(I used to work for a company that made the rubber bushings for alot of auto companies...) the hollow spaces are there to allow the bushings to have a certain rate yet hold everything in place.....by filling the void it will become stiffer yet...(the hollow areas are usually in the x and y direction).....