I have an '86 Mustang GT 'vert that has had the entire suspension/brake treatment done a little at a time since the purchase in '90. The current set up is full MM suspension F/R, Cobra brakes F/R (soon F ST 40 4-pot with 14" rotors), running 18x11s with 315s all around. Street cruiser with some autocross and open track.
At this early stage I would recommend chassis stiffening first (FLSFC, STB, Lower G-load brace), especially if the car is a vert or T-top. These pieces help the vert/t-top cars reduce cowl shake and sagging doors, not to mention helping to remove suspension migration under load. Then I added Flowmaster cat-back, some poly bushings, better struts/shocks and FRPP "C" springs.
Next brakes. I put on rear discs early on using the old Ford M2300C kit which SSBC reproduced. This is a 4-lug kit that uses the same rear brakes as the 93 Cobra and 87-88 TB Turbo Coupe cars. Because my 86 had the small 10" front rotors, I upgraded to the 87+ GT spindles and 11" brakes. Together with the rear set-up, I had the same brakes as the 93 Cobra. The car stayed in this configuration for about 15 years. Motor was stock except for a K&N drop-in filter and removal of the filter box snorkel.
Later I went into the motor by replacing the stock heads with E7s, an upper intake and larger TB from 87+ V8s, and BBK unequal shorts. I also got into more suspension stuff with an MM PHB and eventually a PM3L set-up, front C/Os and 5-lug conversion using the same rear brakes with 5-lug rotors and the Cobra 13" front set-up. Because the 86 as the narrower K-member, I also used 03 Cobra front A-arms.
Now the car is a full-on Pro-tourer/Resto-mod. It has taken until June of 2020 to reach this point.
Good luck guys.