Russ,
Definitely a good idea to upgrade to dual resevoir. I'll be doing that this winter sometime.
Opinions on options:
Option 1: Need a brake booster? Answer this, can you make the brakes lock up if you hit the brakes? If you can, you don't need the booster. Booster can make them more sensitive.
Option 2:Granada is a good idea, or finding a donor 66 disc brake mustang set up is even better. Swap out the entire front end of the V-8 mustang to your mustang. Better all around then the granada, but granada is good. Problem with granada swap now, is they are getting hard to come by and prices are starting to go up.
Option 3: Don't waste your money on a disc brake conversion set up for a 6 cylinder 4 lug front end. They mostly go for $1200. For $700, you can get a good wilwood 5 lug disc brake conversion kit. If you shop around, you can find V-8 front ends for fairly cheap. I located a 65 front end for $100, drums, spindles, idler arm, pitman arm, and drag link. so, for $400 less then the typical 6 cylinder set up, I can now have a V-8 front end and disc brakes.
Option 4: If you are having problems with your brakes, consider a brake inspection. Like David said, a routine brake job at a brake shop can easily run $500-600 for drums set ups, depending on condition. At that point, it's almost cheaper to go with Option 3. After I bought my 65, I took it to a shop for a "free" inspection of my brakes so they could see how bad they were. They ended up being okay, so I waited to save for my disc brake upgrade.
Good luck.
Slade