Unfortunately there isn't any "cheap" way into a performance head upgrade on a LA 318.

The 302 wouldn't be a bad way to go if the heads currently on the motor needed overhauling anyway, but if the current heads have no issues, it's going to be a pretty big expense for minimal gains(unless the 302 heads have a fair amount of work done to them=$$$).

The valve springs on 318's are really quite low pressure, so I would keep the cam selection on the very mild side unless springs are part of the plan.
A nice little cam swap in those motors that can be done while still using the OE springs is a 360 2bbl cam. It's quite a bit bigger than a 318 cam, but works with the OE springs, and the lift is low enough not to cause any retainer to seal issues.