Making Power 318

Hello, I recently just purchased a 1973 Plymouth Duster and it has a 318/727 combo. From what i could tell it's an 87 motor. At the moment it's unbearably slow and I'm looking into modifications to make it faster. I've read about aluminum heads and a cam. The motor already has an Edelbrock performer intake with a 600 cfm carb and from what I can tell just by the sound of the car is a mild cam. I want a way to make more power out of the motor in a cheap way without tearing into rotating assembly. Furthermore, what power could I make simply with heads and no cam? Any help is appreciated, I just want to make the car faster for relatively cheap. Also another random thing about the car is it has a Ford 9" rear end.