The 1973 Challenger had the low-compression, detuned, smog 360 c.i. engine. To improve performance, you should go the tried and true way;

*Headers and free-flowing exhaust
*Intake manifold
*High performance carb (possibly a six-pack setup)
*Hotter cam
*High compression pistons
*Ram air
*High performance electronic ignition and wires
*Stall convertor (if auto.)
*Slicks (for track)