Quote: if your power stops at 4500, that's the problem. heads won't help. Many guys run 500hp+ on old iron heads.
I agree the cam is either fairly small, or way too advanced. Good heads will help, and old iron heads can make good HP, but not in stock form. I mentioned the aluminum heads because the Edelbrock Performer or 440 Stealth heads are an easy and affordable bolt on. Price out the cost of rebuilding stock iron heads with labor, the larger valves, hardened seats, new valve guides, and porting and you will have as much money invested in them as if you just bought the new heads, and the aluminum heads have closed chambers and are less prone to detonation. If you consider a stock unported heads usually only flows about 235 cfm @ 0.500" lift, and on out of the box stealth head flows 270+ cfm, that extra 35 cfm can be worth 70+ HP as long as the rest of the intake/carb/cam/exhaust don't choke the engine.
