A few photos for what it's worth in helping you solve your problems. I have the same 4 wheel disc brake kit on my Dart, but did not purchase brake lines from them. I made my own line from the proportioning valve to the rear of the car (I chose not to use their proportioning valve, bought a...
I think your making this more complicated than it is, think of it as just replacing your existing gauges with another one. The hot or trigger wire from the new gauge can be plugged directly into original harness plug that hooked to the back of the OEM gauges panel, you just need to follow the...