Greg,
I took the brake pedal setup off your 1 1/2 ton, and am planning to do the same thing... Put a dual master for disc brakes in my 1940. If I am not mistaken, on your 1940 the brake pedal pulls a rod and a clevis to another lever that is mounted to the frame. From that lever there is a shorter rod and clevis that attaches to the MC which is basically turned around. The pulling motion on the frame mounted lever ends up pushing the MC. There is force multiplication happening twice in that setup. You have the pedal ratio, and the lever ratio.
I think a standard frame mount master cylinder bracket would work in your case. You would look for a 1 inch bore MC if you are not using a booster. The beautiful part about that linkage is that you could lengthen or shorten the rod(s) to keep your MC accessable.
Your 1940 3/4 ton differs from most of the prewar and decco trucks in that you did not have the transmission mounted MC which is typical on most the light duty trucks I have seen.
Good Luck
John
