Wouldn't the 428CJ engine require the GT components? Competition handling package, etc.
I am not sure that disc brakes were part of the GT package in 1969?
The following is to the best of my knowledge, and I welcome further clarification and correction.
With the FE in 69, you would get the wrap around shock tower reinforcements, and shock, spring, and front sway bar rates/sizes commensurate with your engine choice and how it was optioned.
For example, heavier spring rates for an a/c car vs a non a/c car.
Any 69 Mustang with 428 Q or R and a 4 speed got staggered shocks, and 428 Q or R with 3.91 or 4.30 axle and C6 got a cast iron tailshaft.
Power steering and power brakes were individual options, but were a more popular choice than with Mopars that were notorious for big block power and drum brakes all around.
Most were wrecked before they had a chance to rust.
With regular 69 Mustang, GT and Mach 1, engine/transmission/axle choices determined the suspension setup, not the model.