Why not just use the entire Mustang II suspension as it is proven and widely available, instead of some weird crossbreed?? You can get the Mustang II already set-up for coilovers on the lower arms.
I don't know if there are size or taper differences between MII spindles and the Falcon ball joints, so you will have to investigate this. Back of my mind says they are different, but I am not positive.
The MII also is not stock with 5 lug, they are all 4 lug unless plugged and redrilled or replaced with aftermarket 5-lug rotors.