[cleanspeed1]

Welcome aboard, and as a person who loves the old school, you get bonus points! With a decent amount of room under the hood ( at least lengthwise ), I would look into a couple of things.

Since you are not out for killer speed, and maximum economy, the key would be maximum bottom end torque and nothing torques better than diesel. To keep it 6 cylinders and kind of small, a Mercedes 3.5L inline 6 turbo diesel or the 3.0l twin cam diesel out of the 300 would fit nicely. Going bigger, the 5.9 Cummins would do great, but they are kind of loud in mechanically injected form; the common rail units are pretty quiet. Either way, it'll move that boat down the highway and do 30 mpg or better with a stick and some tall gears.

Gas engine wise, the Atlas engine is not bad, but with all that valve area not great on bottom end torque. You want to cruise, and 2 valves per cylinder and smaller ports will get the job done. Either that or cam those engines differently so that the power is more biased to the bottom end.

That later model 250 inline 6 with EFI is a good start in keeping with the old school flavor, but I don't like the cylinder head. Sissell can do some things with that one, or if you feel like spending some money, get one of those Sissell aluminum heads, a bottom end torque style cam and multipoint EFI. Has a modern fast burn chamber, so that will help with the mpg. Smooth and silky power with some duals running Smiths out the back.

Out of the box thinking: GM 4.8L LS motor, all stock, 5 speed OD, truck intake manifold and EFI, tall gears in the back, carefully tuned. These iron motors can be bought for no money with a lot of life left in 'em and since they do great in the trucks they came out of, your Fleetline will be easy.