I have the same boat, same year (1996 Catalina 28). My rope clutches are all lined up on shiny (no non-skid) raised areas either side of the companionway.
The port side has four rope clutches, which pretty well fill the raised area. The starboard side has three clutches and a clam cleat for the mainsheet.
I use all the starboard clutches (main halyard, topping lift, and occasional use of clam cleat for mainsheet). Of the four on the port side, I only use three (assigned to the genoa halyard, reefing line, and a spare halyard).
Just offering this info in case your boat currently has fewer clutches. My boat came from Catalina with these clutches and they are not through bolted, which would seem to indicate that there's a plate in there.
If you're adding some clutches, I suppose you could carefully drill until you hit metal, drill through the metal and no further, and then tap it. If you never hit metal, then you'll be through bolting for you installation.
Out of curiosity, how many clutches do you have currently?