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.