i hear tell that people have good results supercharging the 4 liter six. id check out supercharger kits and see what kind of power your transmission and such can take currently before deciding for sure on what to do. you will likely get the most bang for your buck with a supercharger. with a special engine build on a six you could easily double the power im sure, though it may be possible to put a v-8 in with out too much hassle.
