Going to replace the master cylinder on the race car and Summit has different bore size master cylinders:
1.031 in.
1.000 in.
1.032 in.
1.063 in.
0.938 in.

My application is 1974 Plymouth Duster with Manual front disc and rear drums. Which bore size is the correct one for my application?

I went with Dr. Diffs 15/16 bore master on my front disc rear drum set up.

http://www.doctordiff.com/aluminum-master-cylinder-kit.html

The smaller he bore the less peddle pressure is needed to increase the pressures in the wheel cylinders Also the smaller the master cylinder bore the less volume per stroke I would probally buy and install the smallest bore M.C. and verify that it was built for drum brake and disc brakes, verify that the reservior for front brakes has the 10 lb residual valve in place under the brass retaining plug and the rear reservior does not have a residual valve Drum brakes usually need a 10 lb residual pressure to prevent the brake return springs from collapsing the brake cylinder in to far, the disc do not have that feature so no residual pressure needed as long as the M.C. is above the wheel cylinders, no gravity drain back.