Don't forget - drag force increases with the square of the velocity - Increasing speed by 33% (60-80) would increase drag force by 77%, or, a doubling of speed would quadruple the drag force - which you have to counter by adding more engine power.
Clutch fan would not be standard in '69 - It will be a fixed blade static fan.
Taking out the thermostat more than likely wont make a difference - except your temp will creep more at idle.
Flushing can remove much of the junk from the cores - or it can compress it into an impassable blob. Since you have the stock top-down radiator, and not a crossflow, solids tend to settle in the tubes and bottom tank, hurting flow.
As for the first theory - that isn't possible - your thermostat keeps the water in the block until it is hot enough to pass to the radiator.
The second - the spark knock is probably from what you said - lower octane gas and no lead. The only way to fix that is to cut back your timing or cut compression with either a bigger cam or different heads/pistons. The first would be easier. Remember, if you're setting it to stock timing specs, you should be running stock grade fuel, which isn't available at the pump. On top of that, you could be running a bit lean, which will never help spark knock and can make you run hot. I'd say this is a real possibility.
10.5:1 is really pushing it with open chamber heads, but it shouldn't be a problem with the correct timing curve, as was suggested. My sister's 302 ran 10.5 compression, and got fairly warm a time or two without detonation. My '67 got up to 250º on the highway when my fanbelt broke, and driveability didn't suffer any and I heard no detonation(for as long as it took to get me to a place to stop) I've also made quite a few drag passes at 200º with no detonation and it has 10:1 compression. (but a biggish cam and closed chamber heads)
I really think you need to rule things out one by one, and start with the ones that won't cost you money
1. Timing is easy to solve, try that first.
2. Check and make sure your thermostat is opening at the right temperature - put a pot of water on the stove with a thermometer and the stat in it and note when (and if) the thermostat opens.
3. Check your fluid levels again (both oil and coolant) and also observe the coolant for any combustion products (indicating a head gasket leak)
4. Check your rad cap. Is it sealing? If it isn't, or if its sealing poorly, you can keep your cooling system from becoming pressurized, leading to steam pockets forming, (and that can lead to detonation easily.) Inspect/have tested/buy new rad cap.
5. Try and richen up the carb some and see if it gets rid of/helps the detonation, without making the exhaust rich
5. Money spending time: Have the radiator checked out at another, reputable shop. Have them pressure test the rad with the cap on it. (if possible) to make sure the sealing flange isn't bent up. Have it flushed again if necessary.
6. If you don't already have one, buy or fabricate a fan shroud - an unshrouded fan is very inefficient, and make sure the fan is within an inch of the radiator (but not too close) buy a spacer as needed.
7. If all else fails, buy a bigger capacity radiator. A 3 or 4-core is pretty much a necessity with an AC system and high summer temps.
Good luck! Just take it step by step and rule things out so you don't have to spend a fortune on it. Keep us posted.
<font size=-1>[ This Message was edited by: thekingofazle on 7/4/06 12:45am ]</font>