Hi Dave V- TSM recommends no larger than 1 1/8" MC bore. I got a '76 Corvette MC for non-power brakes from Rock Auto: RAYBESTOS MC36367. That's what lots of people were recommending around these forums for other brake swaps. I hope it's sufficient. It seems to be what they use in the off the shelf conversion kits. They say "Corvette Style" Master Cylinder.
Greg

1949 GMC FC253 - 1 Ton Dually
1966 Buick Wildcat Convertible