My front disk brake conversion has 2.953 (75mm) pistons and my rear axle (14-bolt) has 1.1875" wheel cylinders. I have not really tested the braking power and hopefully never will need to. Given your intended use I suspect your brake calipers and rear pistons will be up to the task but others with real insight may have a better idea.
I did not have trouble finding options for master cylinders that would fit the S10 booster bolt pattern. In my case the S10 stepped bore master may have been fine but the S10 proportioning valve was not and resulted in rear wheel lock up with medium to high pedal pressures. I decided to jump to the straight bore and manual proportioning valve (for the rear) to avoid multiple iterations.
