Master cylinder rebuild

The dual master cylinder swap is an extremely good idea if you want to live.

Get a new master cylinder (this one), don't futz around with "remanufactured" garbage. It'll bolt right in place of your existing single-pot master, and your existing pushrod will work; see here for how to rmeove the pushrod from the old master.

You'll need a distribution (splitter) block; this can be had used from any A-body with 4-wheel drums, '67-up, but those aren't exactly common in the yards any more, so 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).

All that said, you do need to pay attention to the rest of the brake system to find and fix the leak(s) and whatever other problems might exist.

DOT5 (silicone) brake fluid doesn't attack paint like regular brake fluid does if you spill it, but the silicone fluid brings other problems and issues. I find it best to use the regular fluid and avoid spilling it.