assuming that you have the correct ball joint fitted, and you are using a Mustang spindle, all the ball joints are the same for all Fairmonts 4,6,& v8's and all mustangs from 79 to 93, this includes 4 cyl carb, 4 cyl FI SVO, 6cyl straight and v, and of course the 302 and 5.0, try using MOOG K8259 for example.
now in 94, the ball joints changed for the mustang, and remained the same until '04.
having said so, if you had one of each ball joint in each hand, the original Fairmont ball joint shaft or stem would be skinnier than the 94 & up, the 94 & up would have a considerably chunkier stem area, therefore the fairmont ball joint would use a smaller diameter nut to the 94 & up.
so if you were to later ball joint in the earler spindle, it would not stick thru far enough.
when i changed my my frt end to a 5 lug from a 2000 Mustang, there were two options for ball joint usage, use the later ball joint from the 94 & up which is a perfect fit, or use a special washer from Ford Racing, or you could even stack a bunch of washers up to take up the clearance to allow the nut to be tightened down.
there is a possibility that the new lower control arms have ball joints set-up for the 94 & up spindles.
strangely enough, you can press either ball joint into the original Fairmont lower control arm. the base of the ball joint is the same between both ball joints.
the bonus of using the 94 & 95 and the 96 & up frt end is the braking is better and the wheel choice is almost endless as there are so many 5 lug x 4.5 in wheels out there to choose from.
96 & up let you use the cobra/bullit 13 in rotors and calipers, whith no problems if you choose to do so.
there are so many choices/options, between 4 lug and 5 lug, rear drum or disc, bigger rotors, single/twin calipers, master cylinders, boosters, fuel tanks, engines, transmissions, rear ends,dashboards, suspensions, alternators, seats etc that make these these cars fun to work on, and a lot of people have already done these mods and have the info you may want or need. so many parts interchange from other fox body cars making parts supply endless and readily available. or you can spend the money for the name brand stuff.
good luck with your project.