The '61 upper & lower ball-joint studs are smaller than the later ones (at least they are on the Falcons). I had to machine some adapter sleeves for those too. It was either that or replace the control arms with ones that use the bigger ball joints (like the Mustang ones you mentioned).

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Ryan
-'61 Falcon Futura 2-door, 200ci I-6, 17x8" Bullitt wheels
-'78 F-150 Ranger 2wd, 302ci V8, C4 trans