Because this is such a big deal for cars that have single master cylinders there have been a bunch of articles on this. I am personally familiar with this problem and say it is a real safety issue. Rather than try to give you all the details on this simple upgrade I have a few lists of compatible parts here. Keep in mind this info is also good for most early Ford cars.
For an early Mustang with a single bowl master cylinder there are some quick and easy pieces out there to make this real easy. Simply put it is a realitively quick and economical upgrade. Only a couple of parts ar needed.
The master cylinder swap is truly a pure swap reusing the origonal 65/66 pushrod.
Tubing installation is as simple. Tubing sections and a cap are in the kit. cap off the rear line output of the distribution block and routing the new rear feed to the front master cylinder port and coupling the new line from the rear master cylinder port to the distribution block feed.
I am using this same info to upgrade my 54 Ford. It was a straight bolt-in using 70 Mustang parts. Take a look at them and judge for yourself.
Other master cylinder donors
Some handy brake hardware part numbers
Note that all after market 3/16-inch diameter brake line have 3/8-24 fittings on the ends.
One feature that keeps coming up in these conversion blurbs is the question of wether yo need to put in the appropriate proportioning valve for it to work right. This is the gospel according to Bob: The system will work just fine with the simple setup of front and back plumbing. If one end fails the other will still function just fine. Yes, half brakes, but you will have brakes. They are not cross linked. Hence the two seperate bowls. While the valve does have some engineering features built in to compensate for a leak or failure it is not a requirement. Nifty but not required. On disc / drum configurations the proportioning valve also does some pressure adjusting to give balanced braking. Gotta have one there. Along with a residual valve, but that is another story for another day!