To correct my speedo, I bought a Monty Carlo SS speedo reducer. Goes right by the firewall, and splices right into the cable junction for the head unit, under the hood.
Add or subtract about 15% of speed. You can take apart the little aluminum box and flip the gears if you need to subtract speed. It's about 1/2 the size of a pack of smokes. I bought one from GM about one year ago. Has male on one side and female on the other (Garden hose looking screw on connectors). Also know how to make a new speedo cable out of cheap cables from the store if needed.
Chit, you could put two together and add/subtract 30%. Always sell when done. Takes about two minutes to install.
Looks like your speedo is pressed into tranny housing pretty tight if you did not have the clip holding it on.. Corrosion? Bent ring? (I'm no tranny expert by any means.)
1987 MC SS
Scott