225 /6 at 250HP?

Keep in mind that the slant six is only 225 cubic inches. Doubling the stock HP puts you in the 1 HP/cu.in. range of 225 HP. Doubling your HP is never easy with any engine. To do that will take more cylinder pressure in the form of higher compression or turbo/supercharging. The most cost effective, in my opinion, is turbo charging. There is a lot of information around from others that have gone that route.