You might want to try using the existing proporting valve. I have installed B-car 12" front brakes and Baer 12" rear brakes and the stock valve works just fine.
How do you know if it works ok? Find a lonely straight and wide section of road (or a big parking lot on Sunday morning). Starting at low speeds, hit the brakes hard and see if it stops straight? If the rears are locking up first, you will have a very interesting ride (spin?). Be careful and work your way up to legal speed. You will KNOW if something is wrong! Baer and Wilwood also make a great valve.
You might also want to find a local buddy that races autocross (Solo). They should be able to help you tune your system.
Good luck.