Why are 318's low HP and torque?

I am a new member to the site and I've read this thread over a few times. I have one question but first let me give you a little back ground info. I have a 73 swinger stock 318, with headers, MSD ignition, Edelbrock intake, Edlebrock 600cfm carb, and 3.55 gears, I would like to install a 262 cam, would the 302 heads be a good addition ? I have a chance to pick up a set cheap.