Ok, I figure this out. Before I installed this kit I asked SSBC if I could use the stock Disc/Drum proportioning valve, and they said yes. I wanted to use this becuase it is a cleaner installation and you don't have to try and fit the adjustable valve in-line with the rear brakeline. I did drop the master cylinder size from what they provided (1-1/32") to 15/16" and brake performance was better, but not ot my liking. I was finally able to measure caliper pressure (the calipers have a very long bleeder valve; needed a special fitting) and caliper pressure was only about 600 psi with my wife pressing on the pedal. I swapped in the drum distribution block and installed the adjustable proportioning valve and with the valve adjusted to the halfway point, brake performance was better, but still not completely happy with it. Pressure was up to 750 psi. Limiting the flow to the rear brakes, pressure is now up to 1000-1100 psi and the car stops like it should.

I did check the service manual and it appears the 4-piston caliper brakes use a drum distribution block with a separate proportioning valve, compared to the one-piece Disc/Drum distribution block/proportioning valve that I got from RightStuff. I think the latter may have been used on the single piston caliper cars where the valving is different from the 4-piston caliper brakes. I like the SSBC kit because they are using the original style calipers, they own the rights to produce it and wheel fitment isn't and issue. Anyway, I'm happy with the brakes, but I'll probably keep the 15/16" master cylinder for better pedal feel.

Thanks for the replies and help with this problem.