A little update: I got the firewall shaved of all unneeded holes and converted to a later type of master cylinder. This description won't likely make sense to anyone but 63sbssbbw.
The early dual unit master cylinder has a funny bolt pattern and does not allow converting to a later model disc brake master cylinder like I need. The early master cylinders use diagonal holes and the later use horizontal holes.
The way around this is to weld up the extra hole in the firewall where the clutch pushrod passes through. Then, under the dash, remove the brake pedal bracket and replace it with a 63 and later one. Then, you can drill the holes through the firewall for the later master cylinder and bolt it into place. This allows the master cylinder bolts to thread into the underdash brace as factory and gives full strength of the double wall firewall.
This is the before pic...
Here is after shaving the extra holes and painting the firewall...
I still need to get under the dash and drill through the brace to make the holes though the firewall to bolt the later master cylinder on. The paint is still fresh, so I am going to let it harden a day or two first.