Probably a Dumb 318 Question

Joey N said
What were some of the specs you went with? Heads cam etc..

here's the recipe for 400

https://www.hotrod.com/how-to/mopp-0409-318-engine-build/
and another

https://www.hotrod.com/how-to/318-small-block-build/
and another

https://discover.hubpages.com/autos/Mopar318
you'll notice a trend: closed chamber heads, aftermarket pistons, cam in the 280-ish range and "around" .500 lift.

it's about head flow and compression with a smaller motor. you'll also notice the trend that the power band is narrow and peaky.