how can i get 400hp out of a 318?

There's a reason none or very few of those mag engines tested at westech ever end up in cars to prove out the claimed HP.

Seen two that were tested at westech, then the next day at another facility. Both engines at the other facilities were 10-18% light on the westech numbers. They ran like the lower hp numbers. Westech was infamous for exaggerated numbers. In the words of a respected engine builder in the LA area... big dyno numbers sell engines, regardless of how they actual run. Look no further than the blown 600+ hp 318 that mph'd in the high teens in a 3500 pound car. 475-500hp cars mph there at 3300.

Build one of those engines and find out what they make, more importantly, how they MPH.

Have fun.