I don't think there is that much use for converting a 2 valve/piston engine into a 4 valve/piston engine unless each valve can use separate cam profiles. I say this because it cannot take advantage of the optimum timing of opening the valves, such as having one valve open a bit earlier to create a swirl effect in the chamber among other things. The LT-5 has overhead cams and was designed as a package instead of the extra valves being a super cool add on. Plus, you can make as much horsepower as you want with two valves so I see no real reason to go through the expense to add two more valves/piston on an engine that was not designed for it.
However, I would like to hear from someone else that knows more about this than me, such as Manny, that has experience with modifying engines and if these head would really be worth it. Am I on crack or do I hold some argument?