How much H.P. would a set of headers give me?

Yes, this car is strictly a street car. I used to have a 69 cuda with a 440, I did install schumacher headers on it, but they are pricey and i had to raise the motor to install, as it had power steering. I can't afford that kind of money and i don't want to much hassle. I was thinking i could find a used set of fenderwell headers on ebay and other than cutting the holes wouldn't be to much trouble.