What engine would you build for a Ramcharger?

Assuming your marine engine has the 7 cc dish volume pistons .080 down the hole, the 302's have 64cc chambers (typical real world measurement), and a Fel-pro Permatorque 8553 gasket, you'd have a junk gas friendly 8.5:1 compression. Put the pistons in backwards, ala Uncle Tony's Garage, for some cold weather piston slap but reduced operating friction. Less friction=more power/less load/better mileage. With the lower combustion temperature with EGR, the HEI will be a definite benefit, as well as whole lot of vacuum advance at cruise. I'll have to dig a little more to find out how much cruise timing it will take, but even with the more efficient 302 heads, it's probably going to be upwards of 54 degrees total at cruise speed. It wouldn't be a world beater, but your mileage goal of 18 mpg would be within reach.