Don't bother with internal modifications. You will not get the power you want. I learned that the hard way with my Porsche 928. The car is damn fast but for half the cost of building a totally custom naturally aspirated engine I could have had almost twice the horsepower with a good supercharger system. I know of a guy that is getting 580 RWHP with a bolt on supercharger system on his 928. He has been running it for a while both on the track and off and has yet to see any problems. I bumped the horsepower in my SEC by over 140 hp with my supercharger system. I would NEVER have seen these returns out of a naturally aspirated SEC engine. An engine transplant is a very good idea too. Don't try to reinvent the wheel.