You didn't say what replacement parts you are using.
The Factory Service Manual does contain a procedure for doing this job, if you have not seen it.
IF you are going to use factory body mount parts, recommend you use these parts from the 7B3 police car suspension:
THIS LIST IS THE UPPER CUSHIONS
Qty 4 348080 Pos 1 & 3
Qty 2 330986 Pos 2
Qty 6 330942 Pos 4, 6, 7
Qty 2 488610 Pos 5
Qty 2 14085301 bolt (for #3 position only)
Use the above bolt in place of the longer bolt used with the production #3 mount on the Impala SS and standard Caprice.
Also, for any position that has no lower cushion on the Impala SS (#3 for sure) the flat washer installed in production should be replaced with a complete lower cushion, either the "Mueller" part or another 377801 (OEM production) cushion, which is specified on all lower positions for production in 94-96 cars.
I don't have a solid recommendation beyond what the factory does on the lowers at the moment--which used the same parts on 9C1 and standard Impala/Caprice. I am not sure what, if anything, the Mueller lower cushions will do with 7B3 upper cushions, but it can't hurt, I suppose.