First, one way to repair the mounts for the speedometer is to epoxy
studs (screws with the heads cut off maybe) into the housing.  Then put
the speedometer in place and use nuts and washers on those studs.

On my '85 (two cables with the 02 mileage counter box), if I remove the
speedometer cable from the speedometer itself, it will sometimes be
jumpy when I reinstall it.  I just have to remove the cluster cover and
(while driving) gently reposition the cable until the wiggle stops.

Mike