After doing some research I learned that a power M/C cant be used as a manual due to the rod seat. I found that some of the mustang guys are installing 1985 ford bronco II manual disc/ drum M/C's ( Raybestos MC39401 ). They have a 15/16 in. bore. Does the weight of the vehicle play a role in what M/C can be used ? I would think the weight of a 57 Ranchero would be some what heavier than an 85 Bronco II
There is way more that goes into brake system design then just body weight. The Bronco II is probably a good 1K pounds heavier then the Ranchero. The Ranchero will be in the 3.5K range and the Bronco in the 4.3 - 4.5K range.
I have a basic philosophy that I follow. Detroit pays engineers a butt load of money to make autos work as well as they do. Brakes are probably one of the more important systems. Given the need to have the perfect balance I try to copy exactly what they do and use as many "off shelf" matching parts as possible.
Master cylinder diameter is only one part of the equation to a great brake system. Pedal ratio, disc/drum, disc/disc, size of the slave cylinders, and booster size. All of these are designed around the vehicle and it's intended use. The master cylinder don't care if it has a booster or not. Many are the same size, boosted or manual.
Pedal ratio to master bore is the magic. Pressure to operate the brakes doesn't change much, how it is generated is the key. Volume becomes a factor when larger bore slaves are used. That changes a bunch of the previous factors. So, bottom line, build your system around a system designed as a unit from your foot to the surface of the brake pad.