The hydraboost system does not lend itself to a dual system without lots of changes. You will find the OEM system in good working condition will provide excellent braking at rated loads leaving nothing to be desired. Make sure your parking brake is in good working order as well. Rock Auto is a good source for wheel and master cylinders. I would consider flushing the system (compressed air and brake clean) and replacing all cylinders, even hydravac master cylinder, and service the entire system with DOT 5 brake fluid which does not have a tendency to draw moisture and cause pitting. The rubber lines, front wheels and Master Cylinder, need to be replaced as well. You should not need shoes, if they are oil soaked (wheel seals), just clean them with brake clean. Overall not a bad job and very rewarding.
My project:
https://67-72chevytrucks.com/vboard/...d.php?t=721374
Brake Repair Page 5