Ok fellas...318 help needed , Mpg , reliability and enough power ***for a big boat !!!

I had a 69 convertible boat for 17 years, hence my id. Stock 318 bottom end, Freshened up the heads with hardened seats, Summit's closest cam to a stock 68 340 cam, edelbrock intake and carb, 2.5 inch duals. 727 and 3.23 gears. Excellent cruiser. Sounded great, driving sensible, 16mpg around town and 21 on long runs.