I converted my GTX over to the big 11.75" rotors, big piston slider calipers last year. It also has 11" drums out back.

Left the prop valve alone, even under hard breaking it stops even with no lockup of any particular end. The car can stop fast enough now that it will uncover the oil pickup and flash the Autometer warning light with 20 psi sender which never happened with the drums.

I need to fool about with the master, I think I had a 7/8 unit which was too soft and now I have a 1 1/8 unit which is a touch on the hard side. Have not had a chance to tinker with it further and it does work pretty good. Pads are a Wagner semi-metallic police application.