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

Believe what you like, but 290hp from a warmed over 8:1 318 happened for me 30 years ago.
(Well, actually not quite 30 years ago just yet. The date on the dyno sheets is September 1990)

It’s a “been there, done that....... have the stack of dyno sheets to prove it” thing in my book.

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I think if it had an honest 9:1cr and a real cam it would have easily been about 1hp/ci.
It was .040 over, so it was 324.5ci.
By any chance do you remember the cam used? Hydraulic or solid?