can i use f body spindles for disc brake setup on a 75 duster

They work great, you won’t over angle the ball joints. That whole “over angling” thing was started by ehrenberger with his “disk-o-tech” article, except he didn’t bother to actually check the geometry to see if it was a problem. Basically he was just speculating, and people still pass his speculation off as fact.

There was another article a few years ago where they actually looked at the geometry changes, and surprise!, it’s no big deal. Possibly even beneficial, depending on your car’s suspension set up.

Swapping Disc-Brake Spindles - Mopar Muscle Magazine