I have three Mustangs, all heavily modified to PT style, so whats good for me may not be for you. The Boss 351 has complete ft/rr
suspension from Global West, rollerized perches, subframe connectors, 13" ft rotors by Baer, reinforced chassis, ect..you get the picture. Global West
suspension stuff is not very expensive but much more than oem type stuff. You can buy the cheap Laurel Mountain stuff, but plan to reinforce the arms with plates to avoid flex. If you don't do much HP driving, start with poly in the swaybars and links, roller perches, weld plates to your existing arms, shelby drop, ect. These are many low cost upgrades that will have a favorable impact on your driving experience.
When changing to discs from drums, you usually only worry about backspacing. Some kits increase the track width and push the tires outward and can cause rubbing depending on your tire size.
Stick with name brand brake kits if you do not have the experience to piece together less expensive alternatives and fab brackets. Wilwood and Baer are a good high performance choice, Wilwood tend to be less expensive than Baer but just as good. There are so many other less expensive choices.
www.vintagevenom.com is one of so many and friends have used them with good results.
Here are a couple of more pics for high performance inspiration (351Boss stroked to 393.)