Finding the right dual master cylinder/combination valve and residual valves isn't a big deal either. CPP's website is a good place to start to get some ideas. What i'd do is go to a junk yard, find a truck of a similar size/weight and pull the master cylinder and combination valve. Alternately, go to autozone/napa etc... and buy a dual master cylinder and combination valve from a similar size weight truck from a newer year.

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi