Why are 318's low HP and torque?

I had a 72 318 Dart , all I did was shave the heads a bit , put a holley street dominator , thermoquad off a 74 440 Road Runner with 1/2" spacer , hedman headers , and a cam and 3:55 8 3/4 diff and the thing screamed . Did massive burnouts and ran with 396 Camaros and 400 birds all day .
With the right parts and some luck you'll have a great low buck car .