In stock form the Magnum heads vs 1.88 LA360 heads seem to make 20-30 hp over a similar 1.88 LA 360 headed combo. You should be able to get 400 hp with reasonable cam/cr without moding the magnum heads.
Looking at the MP crate 360/380, I wonder how else
(or more accurately why the factory went the route they did for)
400hp could be made with a different smaller cam and a RPM. I’m not particularly fond of the crate engines single plane, low compression and intake manifold combination choice.