The OP insist on two things he's unwilling to compromise so far 3.21 gears and 400 hp, everything else he's been open to. A 360 magnum will work better under these requirements.
Not that I don't think he couldn't build a 400 hp 318 but don't think it's gonna work to well with 3.21 gears.
Zero deck 318 a set of rebuilt magnum heads, airgap and a 235 ish cam should put him around 400 hp.