According to my measurements of a 69 non-power brake pedal and 70 power brake pedal (shouldn't matter but both for auto transmission cars) and calculations, non-power pedal ratio is about 6 to 1, and power is about 3 to 1. I am not sure if someone makes a 6.25 to 1 pedal that fits a 69 Mustang but a stock non-power brake pedal should work for you. I went from non-power drum to power disk on one of my 69s couple years ago; if I am not forgetting something, I replaced the non-power pedal with a power pedal that came with the kit, enlarged the hole on firewall to install the booster/master. So you may have to install a block off plate to cover the hole I had to enlarge when you remove the booster. Also, you have to install a residual pressure valve on your rear lines if you have drums on rear and Wilwood master does not have a built in pressure valve.