Hey no problem Deanfitz. My car currently has power disc/drum brakes. It was originally just power drum so I had to go on the hunt for the correct booster and master as well. And yes, I converted it myself.
As far as pre gap, it is the gap in between the adjustable nut on the end of the rod coming out of the front of the booster, that mate's up with the back of the piston in the master cylinder. When you press down on your peddle, that rod is what pushers into the master, moving the piston inside the master to initiate your brakes. There is a mere gap in between the two when at rest. That's you're "pre gap". It has to be set, so there is no preload from that nut on the rod to the master cylinder actuating piston when the pedal is at rest. Like mentioned, a pre-load will cause your brakes to drag, get hot and then lock up. Too big of a gap and you'll have excessive pedal travel before the brakes engage. Some systems require a small preload to their design. I would refer to the FSM for the correct setting.
Your proportioning valve controls the amount of braking (to the rear wheels). The factory had/has them and they look like 1.5" x 3" long brass blocks mounted under your master cylinder. A drum brake car also has this brass block, but it isn't a proportioning valve, it is a distribution block. I does not vary the pressure/fluid to the rear wheels. It just takes the two lines from the master and distributes them out to the four brakes. If you run a disc/drum combo or even a great majority of disc/disc combo's you need a proportioning valve to control how much brake force is coming from the rear of the car. Most cars run around 70% braking up front, 30% braking in the rear.
If you plan on converting your ride to disc/drum using all the same parts and pieces Ma Mopar would have used, you could use a factory style brass proportioning valve. It's engineered with the correct amount of pressure ratio for sufficient, safe braking. Those can be found New/RMFG in many places, and sometimes used in good shape. If you are changing up any of the factory variables with your brakes, like master bore size, aftermarket or different body style calipers and hardware, you would need an aftermarket proportioning valve. You changed the dynamic's behind the factory designed proportioning valve and now need to set the correct ratio yourself.
Like mentioned before, a decent aftermarket proportioning valve like Wilwood or SSBC will work great. The have a variable knob or lever on them that lets you control the amount of brake force to your rear brakes. It would obviously need to be mounted on your rear brake line. Lots of folk mount them near the master for easier access. You could also but a combination proportioning valve from Wilwood, which eliminates your old brass block all together. All lines run in/out of it and it has an adjustable knob to tune your rear brake ratio.
Hope it helps.
