The dual master cylinder swap is a very good idea. Get a
new master cylinder, don't futz around with "remanufactured" garbage. Use a Wagner MC71258, Bendix 11323 or Raybestos MC36221. It'll bolt right in place of your existing single-pot master, and your existing pushrod will work. You'll need a distribution (splitter) block; this can be had used from any A-body with 4-wheel drums, '67-up, but probably easiest to get it new from
Inline Tube. You'll need to run dual hardlines from the new master to the splitter block. Be sure to bend at least one fully-circular loop in each of them to let them flex. Steel line is standard and is a pain to work with;
Cunifer is a lot easier (and better).
