Swap or rebuild?

302 heads untouched may give you half a point of CR increase about the same as milling your heads 20 thousands, 1 point of CR is worth about 4% hp so on a 300hp engine thats 12hp, unless your try to use a bigger cam than 262 and or going fully ported theres better places to spend your money. But unported 318 heads will limit your power potential in street trim to about 325hp.