In order to properly replace the upper strut mount whether it is the rear or front stuts, the complete strut assembly must be removed from the car. To get the rear strut out the strut coil spring compressor is not needed, however, you will need it to compress the strut coil spring to replace the upper mount after the strut is removed. Doing it on the car is impossible.
The link below has a tech procedure for both front and rear struts. For a Grand Prix "W" body car but the Impala is identical in the process. I've done a '01 Impala, a '97 Grand Prix and a '99 Regal using the procedure. Take note the rear struts should have a nut attached to the rear sway bar endlink that must be removed.
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