You don't have rack and pinion, you have an old steering box. It is probably worn out. I would look into getting your steering box restored to stock specs by someone like Chockostang, make sure all your steering components are in good condition, then get a good alignment with modern specs. You won't believe it's the same car.
Fox body steering will not work in an early Mustang. There are rack and pinion kits available that do. Many on this forum have them. Do some searching and you will find a lot of threads about this. They get mixed reviews...
"When a woman is a really good driver, she is just about perfect."
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'66 GT Fastback show car, nightmist blue, warmed up original 289 & T-5Z, 3.80 trac-loc, AC, PS, pony interior, Retrosound, rally pac
'66 Coupe driver, Bullitt green, 302HO w/ 351W heads, roller rockers, Holley 600, T-5Z, 3.55 trac-loc, collapsible column, tank armor, disk brakes, shoulder belts
'11 BMW 335i X-drive, 6 speed manual, all the bells and whistles