rear disc conversion

Stick with disc front ,drum rear. Easy to get parts. parts are cheaper and ther are upgraded pads and rotors you can get to improve performance. Also stock spec sized brakes work with factory sized wheels, where most aftermarket brakes require larger diameter wheels and tires = more $$$
To get a drastic increase in baking you would have to step up to a multi piston caliper set-up. The old factory disc was a single piston that pushed from the back side of the rotor only. The multi piston Kelsey-Hayes calipers had inside and outside pistons for more even braking pressure on the rotors as well as more leverage for quicker stops.
Most all modern performance disc brake systems use 4 to 6 piston front calipers and 2-4 piston rears.
Of course the modern performance cars not only have more horsepower than old cars but are also much heavier and need the stronger brakes to stop all that mass.
Modern Mopars weigh over 4 thousand pounds !