Aluminum Slant 6 engine

diymirage said
got a car yet?

those goals may not be easily met in a 50 year old car
then again, ma mopar set those goals when she made the feather duster

my point is, dont look at just the engine, look at the whole package, every pound f car you have to push will run down your MPG
rolling resistance comes into play aswell, making sure the wheel bearings are in good shape and properly greased, lightweight wheels, light tires...all that

Exactly. You would need to run a narrow tire similar to what was factory on the car.
In the late 60's, early 70's I ran a 700-13 tire on the front with about 50psi - not on long highway trips, but it helped with the manual steering and mileage. Not everybody's cup of tea, but rolling resistance, synthetic lubricants and a good wax job are key to your goals.