It's me again, I got so involved with the head gasket thing that I forgot to answer your question. Acid and aluminum don't mix, make sure whatever you use is aluminum safe or you may find coolant dripping on the ground or your carpets.
I recently had the irratic overheating and tried flushing the system, all I did was open a really big hole in the head gasket. The overheating was from an air block in the cooling system. Combustion gases leaking into the water jacket would collect and either block coolent flow or fill the water pump which is not designed to pump air.
The combustion gas will eventually find its way to the top of the radiator and into the recovery tank, which is why I asked did you have any bubbles there. I just got done with replacing the gaskets in my Olds Silhouette 3.4, fun the engine sits sideways under the dash. At least the 4.3 is pretty much out in the open.
If you haven't already done so get a good shop manual, I like the Haynes, I have the GM ($150) but I use the Haynes ($20).
See ya
94