This is not a job for DIY. The internal gears of the spedo are probably stripped. You will need to find a spedo repair shop, remove the spedo head and send it to them. You will be lucky if you can get a 73 repaired.
Looking for a Used Thunderbird in your area?
CarGurus has 24 nationwide Thunderbird listings starting at $13,595.
Search Ford Thunderbird Questions
Ford Thunderbird Experts
Related Models For Sale