A 4-speed transmission might not be so bad at all, since it would be a little lighter and have fewer internal frictions too, but the gear spread and the differential ratio also have a key role on that matter. The close-ratio 5-speed and 6-speed manual transmissions which become prevalent nowadays have been geared to fare well in the EPA test procedures but it doesn't lead to any outstanding real-world mileage improvement.
Regarding engines, even some flatheads could still be kinda efficient in spite of their flow limitations.
Born-again Flathead | Hemmings Motor News