Overheating/proper operation

I recently installed a rebuilt 350 long block in my 78 chevy pickup. New aluminum intake, aluminum water pump, new HEI dist, rebuilt Holley carb, etc, etc. Upon startup it runs approx. 11-12 minutes, then goes over 200 degrees in water temp. Set timing to 6 degrees BTDC, changed therostat to 160, instead of 180. No change. Replaced 3 row radiator with new 4 row aluminum radiator. No change. Installed mechanical temp gauge instead of elec. No change. Removed intake, checked for blockage, reinstalled. No change. If I let it run longer, it gets even hotter, I don't want to blow this new engine. Any ideas? 10-7-07 I just replaced the water pump(again) with an aluminum h/p one, also put the factory six bladed fan back on. Started the engine, let it idle for about 20 minutes, it went to 210 and still climbing, revved it up to 2250 rpm, held it there for about 4-5 minutes, temp fell back to 190, as soon as I let it idle again, back up it goes. I can see the water flowing through the radiator, and I can't find any leaks at all. Rechecked the timing, it has not moved. I just don't know what else to do. Anybody got any ideas? Thanks, DD