Today, dollar for dollar, is the 318 faster than the 340 ???

And a overdrive. With the right combination of LA 318's with 904's getting 20 mpg/hwy. I bet a 5.2 Magnum with a O/D could get close to 25 with the right rear gear and tire.
Since ya put it that way... my 340 with 4 speed and 3:91 rear is a bit like driving a tractor...definitely a "light to light ' car LOL! The car is going back together but when I had it on the road, I could slow down, take a right turn and not have to shift out of 4th gear...I recall it would run 3000 RPM at 65 MPH with plenty of pedal, but its drinking gas at that point.