You set lofty goals for a slant. 400hp is going to be difficult for a slant simply because of head bolt clamping. They just are not able to contain the cylinder pressure needed to make that hp level without serious machine work/modifications. Check out Steve Dulcich’s “quest for slant 6 greatness” on motortrend. His was failing head gaskets around the 300hp mark and was a fairly serious build. I’m not saying it can’t be done, hell I’d love to see a turbo slant make the power. But it’s not going to be a smooth road to get there. Maybe set a more realistic goal (like 250hp) and slowly work up to more hp and find/fix the weak links along the way.