Not so much of a problem in a '67-'70 Mustang if you are converting to power brakes and have a manual transmission and if you are using the factory-style 7" diameter brake booster. The diameter of the 7" brake booster is small enough to clear the clutch linkage rod going through the firewall beside the booster [if you were to use the mecahnical setup].
In a '65/'66 Mustang though, it makes a big difference in what type of transmission you have if converting to power brakes. The stock '65/'66 Mustang power brake boosters have a bracket that stands the booster out away from the firewall. With this arrangement, there isn't enough room between the face of the booster and the back of the shock tower to mount a dual reservoir MC. --you are limited to a single reservoir MC.
A power brake booster that does work in the '65/'66 models with a dual reservoir MC is the '79-'93 2.3L "pancake" 9" diameter vacuum booster. However, the booster covers up the hole in the firewall where the factory mechanical clutch rod linkage passes through the firewall. This means with this booster, you would have to use a cable clutch to actuate the fork on a manual transmission.
The smaller 7" diameter '87-'95 5.0L Fox Mustang booster would also work in the '65/'66 models, but even with the smaller diameter, the booster still covers up the hole in the firewall of the '65/'66s because the hole on them is located much more "in-board" toward the steering column compared to the '67-'70 Mustangs.
With a '67-'70 Mustang, and the stock diameter vacuum booster, you could use the mechanical linkage or you could run a hydraulic clutch setup, or a cable clutch. In the '65/'66 models, if you use an aftermarket or retro-fit Fox booster, you will have no other choice but to use a cable clutch if running a manual transmission. --If the Mustang (regardless of model) has an automatic, the booster type doesn't matter as long as it will bolt up and fits within the confines of the space around it.
