Those are pretty small pistons. If you compare that caliper to the factory A body caliper with a single 2.75 inch piston you'll see that the SSBC caliper has only 75% of the braking force of a stock system.

That means the rest of the system needs to be re-calibrated with smaller components to match. First thing would be to put the smallest rear wheel cylinders that you can find in there and the smallest master cylinder. That might get you back to a properly balanced system.