The power booster and or dual master kits sold by the regular "restoration" parts suppliers for smaller trucks of the same years will fit, then all you need to do is find a dual master cylinder with a large enough bore from a drum/drum brake truck.
I did mine a little different and used a hydrobooster, works off the PS pump, I also have disc disc, so can't help you with a master cylinder. Unless you plan on having PS, then the Hydrobooster may not be as easy as a vacuum booster, the hydrobooster is most often a better system though.
There are two kinds of dual master and or booster kits, some mount with a bracket to the original pedal assembly where the old master was. The second kind mounts it with a bracket inside the frame rail behind the pedal assembly, then uses a longer push rod to reach it. I would use the second type if I were to do it over again.
Grigg
