I have a 64 chevelle that rarely gets driven, it has a 383 with 4 core radiator 180 stat and flex fan aluminum heads, etc. all items are essentially new. Last year even in hot hot weather it seemed to run cool about 1/3 sweep(sweep factory gauge) Fired it up and took it out and its hitting the 3/4 sweep and seems the more you drive the hotter it will try to get. The car has been sitting since last year with no mods...I checked all my valve lash and plugs and pulled the stat, checked head gaskets and all checks out fine. I put the stat (high flow 180 mr G) and put in hot water, water got right to boiling before it opened.. could this be a bad stat since it was nearing 212 before it is opening? Also could I get cavitation in my block from sitting? I run 50/50 antifreeze with a Wix solution to keep cavitation down with the auminum heads and steel block. I am going to replace the stat regardless but should I flush the cooling system as well just to be sure? When I put the motor in and flushed originally it had some white looking build up that I had noticed when I removed the intake... The fluid I have drained looks excellent but could I still have scale build up? Any thoughts or suggestions would be greatly appreciated.

One question, on the Mr G high flow stats, how far should they fully open? This one opened maybe 1/4" at very most

I can give you some advice.Get rid of the junk flex fan.At a cruze the blades flaten out and create a wall blocking the air from flowing through the radiator properly.If you want the best cooling get a clutch fan.I run a hd fan for ac equiped car.I went to a local fab shop and had a piece of diomond plate rolled to make a shroud.It uses some extra horsepower but I have some to spare.160 deg all day long.Looks cool to.

http://i113.photobucket.com/albums/n220/Nikoda2002/Cars%20and%20Trucks/chevellemotor.jpg

Mr Gasket thermostats start to open at temperature stamped, fully open in another ten degrees, 1/4 inch is good. Robertshaw makes all brand name thermostats.
Certainly wouldn't hurt to flush.
White specks is alum corrosion, not much you can do preventing it.
If you can, run a lesser ratio of antifreeze because water by itself is the best coolant.