Although I tend to be a little blunt from time to time bear with me. SCREW the HP, what you need with a big car like the Charger is torque at the rear wheels.

Cheapest/easiest bang for the buck, headers with dual exhaust, 4 BBl intake (look for something that starts making power in the 1200-1500 RPM range) and a lower set of gears (something in the area of 3.50 for the street). Go with a stock or RV type cam for a 318.

When you get through you still have a small block in a big car that will do better, but won't be anything to write home about. You will also have spent in the area of $1500 in parts.

You can spend more, heads, stall convertor, bigger cam even a supercharger etc, and guess what YOU STILL HAVE A SMALL BLOCK.

After you have spent all that money, a 72 Charger with a mildly built 440 will still probably hurt your feelings and have spent a lot less doing it.