Following Daves lead use the floor jack under the shock plate to take some tension off the shackle you are replacing. I would buy new stock shackles or something like the hotchkis shackles for some additional strength. The bushings go into teh frame rail and spring eye, they don't really have anything to do with the shackles. If the ones in yours are not too bad you don't have to touch them. If you do the uppers may be a bitch to get out but once you do clean the pockets with a scotch brite pad and lube them up before trying to install the new bushings. Same for the spring eyes...
If the new uppers are really tight verify you have the uppers and lowers correct. From there use the shackles and a bolt and washers to press the bushings into place.
Now the real caution: A lot of times those shackles were used more as a way to avoid replacing bad sagging springs than as a mod to lift the car... Typically it resulted in jacking up the rear so be prepared to have a droopy rear end just in case...