My setup isn't terribly different from yours. I have C6 calipers and rotors up front with LS1 rear calipers and rotors, 1"master and an adjustable prop valve for the rear, 9" dual diaphragm Tuff Stuff booster. I'm using Hawk HPS pads, which are nothing crazy, and I can lock both front and rear tires with heavy pedal pressure. If you can't lock your tires, something is wrong. When you adjusted the proportioning valve, you should have seen which end locked first and adjusted incrementally until you crossed over, then gotten back to where the front just locks prior to the rear. Are you able to lock either front or rear with heavy braking? Maybe you have the proportioning valve closed off too much such that the front can lock but you aren't getting much pressure to the rear. If that's the case, keep opening the valve until the rear starts to lock before the front and then back off again slightly.
I found using slow-motion video on my cellphone was really helpful to see which end was locking when.
https://youtu.be/_odsr8yN9XI