Speedometer cable

All I can suggest is to clean the square drive port at the back of the instrument. That square port would get packed full of crud ( contaminated grease ). I suppose that's why someone would suggest trimming the cable length there. The cable has a stop collar that sets its depth in the port so if the hole is clear the cable should fit without applying forces on the instrument.
I've handled a number of worn out speedometers and every failure was due to forces applied by the cable. Most were the earlier type. The mating/aligning collar for 68 and later is about a inch long and served a lot better than the earlier type with a collector nut ( same as transmission end ). In any case, The cable really should attach as straight as possible. No angular or linear force on the instruments spindle.
There was a recent post where a member stated a bad universal joint was causing speedo needle bounce. Head scratcher there. I can only guess the entire vehicle was vibrating.