Why are 318's low HP and torque?

-My Dad loved the famed 318.......He said the 360 was a gas hog for the power it produced as the 318 was close to it with a bit lower MPG! Thing is, sure the 360 is bigger, and can have more machine work done to it, but the 318 would haul *** as said if built right and get pretty good mileage still, after the build.

MY biggest question is the 2 different 318's, the LA and the A "Poly-Head" engine..... Which is longer lasting and can have more done to it between the 2? Machine work and all? I KNOW theres very little offered for the Poly engines, BUT I also know they like they're younger brother are damn near indestructible!!!!!

BUT which was the most powerful stock? AND if "lightly" built?