I've seen A LOT of posts about making HP with a 318-So why is it so ???

Impressive to say the least.
I have a hard time with the inexpensive part?

nothing expensive about it at all really except custom slugs. Cheap rods, factory 340 crank with some tricks( not expensive) and second hand heads.
Even used a knockoff crosswinds airgap.
Carb i still have, its on the 360 in my Dart.
Big thing is attention to detail. I ran it with a factory water pump/ fan setup.