That's the thing with poly bushings (and part of the reason why a good portion of people choose not to use them), you have to reuse the original shell...you can reuse them if you extract the factory rubber insert...either that or you buy a new set of rubber bushings and remove it...or make your own shells.
The problem that can arise with using poly bushings, as explained elsewhere in this site, sometimes the bushings are too big to reasonably fit the original bushing shell due to mfr'ing variance and sometimes have to be turned down to fit correctly...or so I've read. I know there are people on here who've broached that subject before...some careful searching should give you a better answer.