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

If your car has a good running 318 or you already have one to put in your car, definitely worth small ish cam 4bbl and headers or just duals 250-300 hp, after that it's hard to do more for most, most aren't gonna port heads and swap to higher cr piston so limits what can be done and if your rebuilding it is worth your time to dig up a 360 block but if not no reason you can't make power that you could do with an average built 340 just at 300-500 rpm more. Could also throw on aftermarket heads on a stock short block with a xe268h ish cam and airgap and come up with decent power and have the parts for a future build.

But I agree the best bang for the dollar is to find a running 5.2l/5.9l Magnum and add 4bbl for 300 + hp add cam and headers for 350-400 hp.