The '73 and up a body upper control arms take the bigger ball joint size neccesary to run a '73 up a body disc knuckle. You can also use other body style knuckles but some people feel it could cause bump steer issues. If you do go with the 73 up style discs be ready to switch to 5 on 4.5" bolt pattern wheels. You will also need a new m/c and proportioning valve. I went with an adjustable- the stock setup is known to lock up the back first which you don't want.

You can keep your small bolt pattern and small balljoint upper control arms with a Kelsey Hayes four piston caliper setup. It came stock before 73 but it can be tempermental- the pads tend to wear grooves making them not slide properly on their shafts. Also even though the parts are very similar to a 60s mustang setup they are rare and expensive.

On my '73 dart I went with big bolt pattern stock stuff (had to change ucas) and it works great. I changed the rear axle to match the bolt pattern. On my '72 h code demon I went with 1990 chevy celebrity brakes with a scarebird adaptor to mount the calipers. Kept my 5 on 4" rallys.

Good luck.