Small Block engine builds?

[Axels73Charger]

Well I figure theres enough people building small blocks for performance and gas milage I thought I'd start asking questions. Im trying to buy a 73/74 Charger and Im trying to get a 318. Now I've been reading about ways some people have been building them to get about 300 hp. That sounds cool but does that kill the gas milage that much? I've heard from some people that they've gotten 24 mpg with a 318 in a Charger. Well thats cool! But if you put 360 headers 4 barrel carb and new cam wont that kill it by alot? I'd like 300hp or so. But if im only gunna get 15 mpg im not gunna do that. Now I dont think it'd kill it that much but if it does...

I know with the cam it's about rpms and what ever. So if you got a mild cam would that work? Maybe to still get some good hp and mpg would 250-300 hp kill alot of hp? Also if I got a 360 could I do the same kind of things to a 318 and get more hp?