I don't know of a spacer. The brass collar is shown on the instrument. There's a stop collar at about 3/4 inch from this end of a speedometer cable. It should be affixed to the cable.
If this is a 150 mph unit, or if you feel you just have to have the trip clock, you might send this one out for repair. It's broke pretty badly so repair will be quite expensive. A good used speedometer along with brand new cable would cost much less than repairing what you have.
Odometers without trip clock are more dependable.
If I was going to buy a used speedometer for a rally dash it would be 68 and later without trip even though there are slight differences in how the numbers on the screen look.
I would need to replace the speedometer cable for a couple reasons.
(1) The 68 and later cable to instrument attach is different from 67 and earlier. 68 cable wont have the threaded collector nut at instrument end. These have a plastic sleeve with clip that better aligns cable to instrument.
(2) Forces from the cable are what kills these instruments. So a new cable is highly advised with any instrument.
If you stay with this earlier threaded type, best you can do is snug up that nut to hold the cable aligned as much as it will.
One more lil' tip... Sellers of used speedometers typically photograph the instrument in a leaned back position, resting on the input shaft. Wear in the instrument causes the magnets to draw the needle away from '0'. If the needle lifts to more than 5 mph, continue shopping for better.
When new, bias force on the input shaft wouldn't move the needle at all.
