It might be as simple as the speedometer cable housing having become disengaged from the rear of the speedometer head. It's a slip-fit, possibly with a plastic retainer, I believe. You can access it from underneath the lh side of the panel. Just find the cable housing where it comes through the firewall, above the steering column area, and see if you can push it back into place.
Not sure how to get to the clock for removal. Inside the clock is a set of contact points, not unlike ignition points. With time and age, they can become corroded and need to be cleaned. You might also apply some light oil to the moving parts, for general principles when you get it out of the dash and disassembled.
You'll probably need a FSM or similar. If the are's a good exploded view of the instrument panel and its parts in a '69-'73 C-body parts book, it might offer some insight into how it all goes together.
At the factory, the instrument panels are fully assembled when they are installed into the vehicles. BUT, "in the field", removing the instrument panel is ONE thing most techs do NOT do, usually. Almost everything can be accessed form underneath and individually. Some items come out from the front. This keeps their presence in a defined area of things, with fewer issues should they forget to hook something up and then have to take it all back loose again to fix that item.
Back in the '90s, the T-bird/Cougar cars of that era had heater cores that would fail earlier than normal. The dealerships charged about $2K to do that job, parts & labor, as you had to remove the complete instrument panel just to get to the heater core. A friend had an auto supply and a mechanic that worked for him. One of his customers came back lamenting the cost of the heater core replacement at the local dealer. Mike though "No big deal, surely we can do that job cheaper." So they got the car in their shop area and commenced to find out why there was so much labor involved. "Never again", he said later. We can get it all apart and fixed, BUT should we forget to hook something up or smash a vacuum line, or something, that would require another removal operation to fix that issue. So they refrained from doing repairs that sounded like they could do them easily, after that.
Hopefully, you're in good enough physical condition and cardiovascular shape to push the front seat all the way back, then hang your knees over the backrest, as you rest your shoulders in the footwell area to do these things under the dash? Have your tools easily reachable for best results.
Just some thoughts,
CBODY67
