A cheap conversion would be great but personally what I did is I went and picked up a set of axles out of a 82 truck that someone was parting out for only a few hundred bucks. Granted a axle swap is a lot of work but I needed a cheaper solution to get away from my widowmaker rims than spending 2500 dollars for a set of aluminum rims that I didn't really like and my brake system already needs rebuilt anyway.

On the topic of a master cylinder solution through several weeks of research I found that a 2005ish npr or w5500 cab over truck has a vacuum system and the mastercylinder is a dual resevior deal with a similar bore to the power brake master in the truck the axles came out of so that is a possible solution for upgrading a master cylinder

1967 Chevy c50 327 w/ sm465 5000Ib front 15k 2 speed rear w/ 22k springs, single axle, hydraulic brakes