here's the recipe for 400
[URL]https://www.hotrod.com/how-to/mopp-0409-318-engine-build/[/URL]
and another
[URL]https://www.hotrod.com/how-to/318-small-block-build/[/URL]
and another
[URL]https://discover.hubpages.com/autos/Mopar318[/URL]
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.