Interesting things under the hood of Macs 63 GP..
 

Look at the photo and try to pull your eyes away from the OHC 421 and notice the brake booster.. Looks like Hydro Boost, a dual master and a proportion valve but the car has 8 lugs. Interesting..

This has always been one of my dream cars..

The picture was too small and too dark for me to really see the details. But I can certainly see why a vacuum booster would not fit with the tall cam covers. So a hydro-boost is a natural to retain power brakes. As far as the valving, with a dual master cylinder a pressure differential valve/switch may have been added to light the "brake light" if there was a leak in the front or rear system. It's possible a full combination valve may have been added to get some rear proportioning as the hydro-boost would easily have enough boost to lock the rear brakes. Mac was an amazing guy and I bet there were many other thoughtful "tricks" in the car.

If you want this car and have $$ I can connect you with the current owner. I told him about your post re "dream car" and he is willing to sell but it won't be cheap. The overhead cam engine is no longer in it.

the motor for this car is for sale in detroit...or was..i havent checked on it...

some how someone swindled the motor away from Mac is the story..i dont know facts i just found it for sale...and others have elaborated it was a real swindle
https://forums.maxperformanceinc.com...05#post6050005

Seems like the value of the car would be in the motor. Other than that it's just a very nice 63 GP that was owned by a GM engineer.

I was fortunate to see and hear the car together and in running condition. It was a strikingly beautiful car. Seems like I remember the engine being on E-Bay or somewhere like that for an astronomical price. Lottery type money.