I have 66 mustang 4 lug front spindles

I think they will work.
If they wont its only because of the ball joint pin size is different.
What I mean by the balljoint pin, is the tapered pin that has the threaded tip for the nut.
The way to check is to measure the pin size and compare them to the ones you have.
If they are not the same it may be possible to find balljoints with the proper size pin that will fit the mustang control and A arms. (somtimes a pain in ****).
With that all said, the other part of the conversion, Im sure you know,is the master cylinder and the proportioning valve for disk brakes.
you can also use a manual adjustable proportioning valve if you did not want to use one made for disk brakes.