Last time I looked at the SSBC kit they were using a Mustang 4 piston caliper with small piston calipers. The pistons were too small and didn't generate much brake force.

I learned the hard way back when I was doing brake kits that hardly anyone understands how brakes actually work. Most people know that if you use a big cam with a tiny carb you have mis-matched parts and the engine doesn't run right. But I found that almost nobody understood that if you put tiny calipers and a big master cylinder together you couldn't stop the car. For some reason people think they can mix and match brake parts and it will work.

The brake system has to be properly balanced. Pedal ratio, MC bore size and caliper piston size all have to work together or else you have a mess. A lot of the disc brake kits available for classic Mopars do not work because of this problem.