302 heads for 318

The cam is the weaks part for performance in a 318
A cam will make a huge difference even with a 2 bbl because even a 4 bbl is only used as a 2 bbl 99% of the time. A stock 2bbl will be a restriction (mainly at wide open throttle) and cost some HP over a 4bbl but will flow enough fuel and air for your mild 318, my cousin's stock car's with a build 360 runs only a 500 cfm 2bbl (class rules) making close to 400hp. Side note 2bbl and 4bbl cfm ratings are different so a guesstimated 800 cfm 2bbl equals 600 cfm 4bbl.