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

Good question. (1/4 ET) I think a good and fair guess is excellent inputs into a computer calculator. Try the Wallace stuff. It does assume the car is hooking and booking.

3.23 & A 26 tire would be good. I’m also not fond of anything higher than 3.55’s for an actual everyday driven street car. My ‘79 Magnum has 3.55’s & a 245/60/15 which is about 26-1/2 or so. No cam in the 5.9 power plant though. It’s just a few RPMs higher than the 3.23’s it replaced. The bearing for the pinion let loose on the 9-1/4. BOOM @ 65 & lots of smoke in the mirror! LMAO!

I had 3.23’s and what ever the stock tire was for a ‘73 Cuda w/14” wheels was. They were small tires at IIRC, 225/60/14. I forget. Tiny suckers looking like a doughnut to eat. It did tach rather high for the Hwy. I thought.