I have to wonder if a stock 318 will handle a 4-bbl at all? At minimum, you'll need to dump the restrictive single exhaust or it'll choke the thing to death. Honestly, the 4-bbl is the final concern with a 318. You gotta be able to breathe deeply first.

If you have a 68 motor, don't expect the exhaust valve seal to last long on unleaded fuel either unless it's already been hardseated at some point. What's your budget? If it's low, any set of junkyard 360 heads, while not as good as Magnum heads will flow well on a 318 for cheap. Buy a Fel-Pro top end gasket set, add a double roller T-chain, a medium cam, and headers too. Then add a 600cfm spreabore carb for crisp response. Anything more is too much on that little thing. You'll eventuallly have cam bearing problems, but that's about as low budget as it gets to have gains from your efforts on a small engine. 318's do make significant gains from those little things tho.