I did an SSBC conversion on my 65 Fury with out issue but I tossed the "jam jar" master for a dual. I'm pretty sure the single master doesn't put out enough volume for disk/drum so the pedal bottoms out. There's nothing wrong with their conversion other than their lack of information to the end user to make it function properly. You'll need a dual 1 and 1/16 master and an adjustable proportioning valve for the rear circuit ( half way open or closed is perfect - 3 turns I believe. ) The pedal may be hard to push so usually a booster is used with that bore of master.
If you had manual brakes before the conversion the pedal ratio may be off and you'll have to mount the pushrod a little lower on the brake arm by drilling a new hole for the end of the pushrod but not so low that the rod rubs on the firewall opening or entering in the master. Many complete systems ( master and booster ) use a slightly tilted bracket to circumvent the issue of pushrod clearance because the rod lifts and lowers slightly as you push and release the pedal. You may also have to increase the length of the pushrod but these are available as a complete kit that you size yourself. I don't think the stroke on a single master is long enough for a dual master. You won't know until you install a dual.Make sure under all circumstances that you bleed the system a couple times - mine took three bleeds to get the air out of the rear circuit and then the pedal firmed up, With an adjustable proportioning valve you can shut off the front and rear circuits one at a time to bleed each one separately. Like I said earlier, if there's no air in the system, the pedals to the floor and the rear shoes are adjusted up, you just aren't displacing enough brake fluid to all the wheel cylinders.