I don't see any evidence a 318 needs a lot more head, carb, cam etc. To make 400 hp.
Look at those 400 hp 318 combos, it's basically headers, airgap, 750, 224-240 cam, 9.5-10.5:1 cr, heads that flow 220-230 cfms, you look at 400 hp 340/360 builds pretty much the same recipe.