If dialing the rear brakes back with the adjustable prop valve helps then your problem is almost certainly in the rear brakes. You said Ford rear brakes? What size are the wheel cylinders?
And for the op, i agree with PST. The SSBC kit seems to have generated a few threads on here lately with issues.
You have to consider the whole system when you start changing caliper piston sizes, master cylinders, wheel cylinders etc. If you mix and match a bunch of different stuff it's pretty easy to end up with the wrong ratio and poor performance.