how can i get 400hp out of a 318?

It's no harder to get 400hp from a 318 than a 340. They even behave the same way. The issue is peak and peakiness of the torque curve. These two tend to be higher for peak, and peakier in terms of the overall operating range for a high horsepower, short stroke engine. But it's not hard to do. The factory 318 heads are a little restrictive for that goal, but any 340/360 head, or better yet the Magnum based aftermarket heads, performance pistons, and a hydraulic cam and it can be done. It will just need to be paired with at least a 3-series rear gear and some sort of convertor, because the torque will be higher up the rpm range, and not as flat a curve.