Odds are you have a blown head gasket....

Went through the same thing with my 413 this idiot "pro" took 5 months and ultimately most of $10K building....

Have now blown hg twice - between 3&5. First time was caused by a misfire when the idiot's wrongly selected bronze gear ate away. No heating prob then.

Second was the guy that fixed the remaining prob (a heavy deiseling under any load and a weird lack of afterfiring even when severely retarded). He didn't torque the AARP L19 studs enough - THAT caused three incidents of superheating, after some decent running.

Heads only took slight damage each time - and were checked by a DIFFERENT machinist the second time for hidden cracks, warping, etc.

I hope it's over now. It made it about 500 miles so far and I'm about to take off the headers to retorque it to specs. Fingers are remaining crossed while I wait for my tranny dude to finish ripping apart my tunnel to plant the gear vendors and for my Dewitts and combined 6500 cfm of Zirgo fans to arrive and magically install themselves.

Anyhow I hope that isn't your prob. Of course you check thermostat operation (I have a restrictor plate, not a thermostat). Make sure your lower hose isn't collapsing. Hit your radiator with some carb cleaner at the car wash, then degreaser/engine cleaner/presoak and then with the pressure washer.

Of course you have to "burp" the air out of the system. Mine's easier because I have a nifty intake mounted water neck add on to make it the highest point in the system. Before I'd park the car on an incline, run it until warm but not too hot (32 secs), crack the radiator cap and squish the upper hose to get all the air out, then adding more and more water until it wasd through. Use hot water and you MIGHT get around your thermostat opening and closing. You also hhave to make sure your heater valve is open so there is no air hiding in there. (I have manual cutoff valves and the restrictor plate so all of this is easier.)

Because I'm super allergic to normal ethylene glycol af, I use the less efficient propylene glycol stuff. While it's true water conducts better, the unpressurized boiling point of 50-60% of that af is 250-260 deg. Adding 3 deg for every psi on your cap and you can get to 90+ degrees higher bp than unpressured water.

If you notice a real surge of coolant blowing out the overflow, assuming your cap is reasonably OK, that's most likely cylinder pressure leaking into the water jacket through a blown hg, warped head or engine deck or some such.

There's a like super minor chance you could cure such by retorquing the heads, especially pushing it 5 pounds over torque and doing it when the engine is at least warm, rather than cold.

A compression test when hot MIGHT find such a prob - or it might be too erratic for even that to easily spot it.

Try everything else first, but there's a fair chance you'll need to rip off the headers, the intake, pull the heads, get 'em checked for warp and put on new gaskets.

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Flex fan or clutch? Toughy. I have flex fans and no clutches on anything. But flex fans basically pull very little air past 3-4K rpm as the blades are designed to flatten out to run super high rpm. They are noisy as hell too. New fan clutches are fine I suppose - I just have the same perspective of a person who only has ever owned clunkers so they always go out. The vette seems to like a weird sized one.....17" isn't really big enough and 18" won't fit. Do NOT run a factory clutched fan without a clutch at any higher speed! If you do pick a flex fan, get the super high flow ones - some are not (more blades, bigger blades, more pitch, etc.) Electric is by far better though, especially for low vehicle speed/idle operation - especially in vettes with really poor air flow. I never see specs on manual fans, but SPAL fans can exceed 3K cfm....Zirgo nearly 4K.