Once you've exhausted other possibilities, you might consider buying a new steering box. I think the company is called Flaming River or something like that. I know they make boxes for the 65-66 models and I think they might for later models as well. It's just like many other parts on the old Stangs, they just weren't made to last 30 plus years.
Stefan
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66 Standard coupe
200 inline 6 - Pertronix ignition, Flamethrower Coil, K&N, Clifford Perf. 6 into 2 headers, Holley carb, three row radiator, Firestone Firehawk 215/60R14's
96 GT Convertible
Bosch Platinum +4 plugs, B&M Ripper, Borla Side exhaust,
K&N, removed air silencer, Tokico struts/ shocks
Next up: 3:73's, high flow H-Pipe
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