289 cam ?

I have a stock 289 with tri-y headers, a one inch spacer on the stock intake manifold, and a holley 600. I'm pretty sure the heads are stock. the car is also a four speed with 2.80 rear gears. My question is, with keeping the stock heads what is the best cam I can get for the most horsepower and torque? This is not necessarily a daily driver but driven often in the summer. A rough idle would be nice but not to the point that it stalls out at every light. I would love to build it up to 375 hp, but that isn't going to happen with stock heads and intake. So how much horsepower do you think I might be looking at by just switching out the cam? Thank you for your replies.

Is there another set of heads out there that would give me an increase in performance without spending alot of money? Lets say under $250.
What might be a good intake manifold to replace the stock one with?
Any advice will be greatly appreciated.