64Belvedere and Dusta hit the nail on the head! You have to big of a master cylinder. Small bore = more pressure. FORMULA FOR OUT PUT PRESS OF MASTER CYLINDER
You want a 1" or 15/16" master cylinder, since the wheel cylinder in the back are 15/16 you can not go smaller thant 15/16 bore master cylinder, or you will run out of brake fluid.

There are boosters made for the Mopar they cost under $250 for booster, bracket, and m/c.

If you have a vacuum problem and still can not get you car to stop. Do the following:
1. switch to a 4 piston fixed caliper up front;
2. replace the 15/16" wheel cylinder in the back with 7/8 or go to a 4 piston 1.38" piston caliper set up.
3. use a 7/8" master cylinder this will give you an output pressue of almost 1,000 psi with a 6:1 pedal ratio and 100 pounds of applied pressure with your foot.

Normally takes 1,000 to 1,200 psi to lock up the front brakes.

BTW: Here's a picture of a "Z" rod bracket for the Mopar Booster. But dont waste money on a booster without first checking your vacuum.

Gator, just hang a set of these on your '65 like I did on my Coronet! These babies will stop that car.

andy, i'm thinking about it! do you use a special porportioning valve? i wish that it was possible to keep the lines and split at the master cylinder that have now and just mount discs on the spindles. it looks like SSBC has spmething that might bolt up.