To measure the bore diameter, remove the master cylinder from the booster (often you can do this without removing the brake lines) and with a caliper measure across the hole that the master cylinder piston rides in. That is the bore of the master cylinder.
For power brake systems on hot rods, 99% of the master cylinders out there are either 1", 1 1/16" or 1 1/8". GM did make some step-bore master cylinders, but they are uncommon and not typically used on a hot rod.
Many of the commonly used master cylinders that come with the aftermarket kits have different bore diameters available so you won't have to redo your brake lines to swap it out.
Keep us posted on how you are doing.