Hey 66DartSedan,
I just did the dual pot master upgrade with my 64 Dart. I was able to accomplish this task with much help from this forum. Before you call and start asking for parts, you need to determine how deep into the project you want or need to get.
You say that you have leaking brakes. Does this mean you have a leak at a wheel cylinder, or did a line break and now you have no brakes at all? If you have a leak at a wheel cylinder, you should check all of them.
When I brought home my new ride a couple of months ago, I knew in advance that I would be replacing every part of the brake system. So, I ordered all 4 wheel cylinders, brake line similar to what Slantsix Dan recommended (S.U.R.R. I believe), a dual pot master cylinder, fittings, all the brake components (shoes, and new hardware kits), dual master conversion brass block from Inline Tube, and some Dot 4. I took one day and went through the entire process. You could take less time and throw in the dual pot and use the existing brass block with a plug in it for the original rear line fitting. Then they run the rear line direct. There was a great article that I read a couple of times at earlycuda.org linked here Early Valiant Barracuda Club - dual master cylinder. If you have never done this before, you should wait to unbolt the existing master cylinder until you have used the brake pedal to pull the push rod out. A quick hard pull is all that it takes to get the original rubber grommet to let loose. Much easier than putting the single pot in a vice and pulling. Don't forget you will have to adjust your brake light switch when this is all said and done. Search dual master cylinder in the top right of this page. Many great articles.
Good luck and enjoy the better braking experience!
Doug