Get 94-95 spindles, and then buy 4 5/8" spacers at Lowes, and put two spacers on top of the spindles after it is on the ball joint. This spaces up the castle but so the cotter pin is properly alilgned with the hole. (Or go to AP store and buy 94+ ball joints. Then you won't need spacers. The ball joints may cost you up 50 dollars or more depending on brand. The spacers will cost you under 6 dollars.)
Then try and see if they have 99-04 Mustang V6, or V8 brakes. The PBR's are way better than the 94-98 calipers because they are lighter, dual piston, and look better too. You will also need to get the flex hoses and the bracket that mounts those to the framerail off the car you're getting the calipers from. You will also need to buy at an auto parts store a reducer union for the passenger side brakeline. It needs to go from 1/4" to 3/8", or vice-versa.