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

Nv3500 would be my choice. For reference, my 03 dakota 3.9v6 with nv3500 3.23 gears 29” tires and weighs 3800 empty, I get 20mpg at 70 all day. A 318 would move my truck around even easier and probably get the same mileage. I’d keep the 318 and put in the nv3500. Stock cam or very small upgrade, 600-670 cfm, headers (short of long tube) 2.5” duals and you’ll cruise and be plenty happy with decent mileage