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

I agree that you wouldn't get 450 lbs-ft out of a 318, some 408 don't make much more than that, unless your talking some crazy highly tuned build like a tunnel ram,W2, big solid roller with 15:1 cr 318.

Most regular guy builds with street cams and cr are gonna be between 1.1-1.25 lbs-ft per cid. so 350-400 lbs-ft and most wouldn't see 400 lbs-ft.
Torque mainly depends on a NA engines displacement, VE% and CR. Torque very by a pretty narrow range around 1:1 to 1.45:1 lbs-ft per cid. I'd use 1.15:1 as an general average build estimate.