An awful lot to read here, but I haven't seen anything about brake pedal ratios.
I have seen a
post on this message board about a guy that cut off a second pedal arm and used it as a "bellcrank" to maintain the the correct pushrod angle.
I'm going to be using a reman hydroboost from an 85 C30.
The two bolts on the plate that it is mounted to seem to be a direct fit to the firewall. See attached image.
Now I have found your "adapter plates".
My 1966 GMC was manual drum brake, which had a higher pedal ratio than what you should have for "power brakes"
Does your kit include provisions / instructions for re-drilling the pedal and moving the pushrod lower? Do you use the OEM 1966 pushrod?
I want to get this right the first time, and everyone says that you are the guy to talk to.