Sounds like a lot of work for not much of an improvement (if any) in power, mileage, reliability, etc.
The 4.3 is a good engine, but I'd rather have the 4.0 personally.
That said, the 4.3 is basically just a 350 missing 2 cylinders. There are performance parts out there for them. For the time and hassle, why not just do a 350?