At nauseum..318 intake manifold to 340 or 360.

Maybe it does have 340 heads, maybe it doesn’t. It’s an easy tagline the seller can toss out w/o really needing to prove if the potential buyer doesn’t ask to pull the valve covers before buying it.

Regardless what heads your 318 has, upgrading to a 4bbl will make a difference. Also consider adding a dual exhaust and recurving the distributor as it’s likely set up for a smog advance.

I replaced the 2bbl on my ‘75 318 with a 4bbl and did a summary here: LD4B installation on a 75 318


Oh, and post pics of your Duster!