Clogged Radiator - 2002 Tahoe

I have a 2002 Tahoe, 5.3L Vortec Flex-fuel with 75K miles. A couple of nights ago my wife stopped at the end of the driveway (everything was fine) to get the mail and paper. By the time she got back in, the Check Coolant
message was on the message center, and the temp gauge was pegged. She parked it in the garage until I got home 10 minutes later. When I looked at it, the expansion tank was full (overfull). There were no leaks.
I did get a little hasty and opened the cap too soon. There was still too much heat and too much pressure, and I got a full boil-over geyser from the tank as the coolant in the block boiled (pressure was keeping it from boiling). Knowing from past experience that refilling a boiled-over block can cause more damage, I waited a while, then refilled with water and ran the engine. Seemed fine, but overheated again after 10 minutes of idling.
After picking up a thermostat, water pump and fan clutch at the parts store, I discovered it was none of these, but that the radiator is completely clogged. I noticed while disconnecting the hoses to check the thermostat that coolant didn't rush out of the lower hose like you would expect from a full, open-vent radiator. (Yep, this engine has the lower hose connected to the thermostat housing, not the upper) Connected a garden hose to one end, with 40 PSI got a slow flow out the other end.
Could this have been caused by switching from Dex-Cool to standard ethylene glycol coolant? I'd heard so much negative press about Dex-Cool (in this forum) that when the warranty ran out two years ago, I completely flushed the system and refilled it with EG. That was at 36K miles, two years ago. Last fall at around 55K miles I flushed it again and refilled with EG again, and also replaced all the hoses (My preventative program is flush and replace the stat every year, replace all the hoses and belts every three or four years).
This seemed to come up pretty suddenly, as we have never noticed any cooling issues before now, but this thing is hopelessly clogged. It is a single-row aluminum core, will probably look for a 2 or 3-row brass core as a replacement.
Sorry, I run on too much here. Bottom line: A. Could switching to ethylene glycol have caused the clog-up? B. Would it follow that other aluminum passages (heads) could be suffering the same fate? C. Is there likely to be much damage to the aluminum heads after the overheat (brief) and boil-over
Thanks in advance for you input,
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Quick update: I took one of the end caps off the radiator, and found a thin coating of almost white, soft material on the aluminum parts. I could wipe it off with a finger, or a strong blast from a hose. It sort of looks like mold. There is a little of it on the plastic end cap, and none in the hoses. The coating is thick enough to just about close off the cores. I tried pouring a small amount of radiator flush on a spot, didn't see any chemical reaction, didn't seem to open up the core (though without heat and high pressure the flush may not work as well). Sure looks like corrosion, but I wouldn't expect corrosion to come off so easily.
Any thoughts?
Thanks, Sandman
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I had a race car and put 40 below in the cooling system. It's like water wetter. It did just what you're describing. I used every radiator cleaner I could find and got most of it out.
Al
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