The only decent kit that I am aware of is this one. The only problem with doing this is that the more you build up an all motor high horsepower 4.3, the more that the power moves up the RPM range. I have seen drag prepped 4.3's use adapted lunati cranks to punch the displacement out to 5.0L and pump up the power to 500hp, but even something that extreme is kinda weak in the low rpm range. Just an fyi. That is why I say to use a forced induction setup from someone like http://www.machperformance.com