After flushing my radiator (327 Gen-I, 1968) with new coolant, I was burping air from the system by using one of the burp kits fastened onto the overflow/reserve tank. My radiator has no cap, only on the metal reserve tank.  I let the engine warm up and bubbles continued to surface in the open funnel. I thought I should let it run until bubbles stopped. But as the engine heat gauge reached 210, the coolant boiled over and yes, made a HUGE MESS all over. I confirmed afterward the thermostat did open. 

What did I do wrong?

I'm guessing I should have turned the engine off well before reaching 210 degrees.  Probably since the system wasn't pressurized (cap wasn't on the reserve tank), the boiling point was reached.  Maybe I should do 2-3 cycles of bringing the engine up to 180 degrees or so, then let it cool down before duing another cycle.

Thoughts? - this is the first time I've flushed the coolant since I've had the car.  THANKS

On small block Chevys, there should be a small vent hole in the thermostat.  This allows air to vent past the thermostat.  Otherwise, there can be an air bubble under the thermostat preventing the thermostat from opening in time.  The engine will then overheat, blowing water out.

If there isn't a tiny hole just drill one.  1/8" or less is more than enough.

I don't remember off-hand if the small blocks had a heater hose return to the overflow. If yours does, I would suggest turning the heater on while purging. This will circulate coolant full-time, regardless of thermostat operation. Making sure the thermostat has the bleed hole is a good call, since some off-shore ones do not. The ones that have the little flipper valve thingy in the bleed hole need to have that flipper thingy removed to allow better bleeding. I have been known to drill several 1/8" holes around the perimeter of the t-stat to promote full time coolant flow. We don't really need a fully blocked system here in Texas, plus if the t-stat valve happens to stick closed, it helps keep from overheating due to no coolant flow. 

I feel your pain!  

I'm not sure of the proccess for filling the rad is the same in '68 as it is on my '78, but we recently had to fit a new radiator, so had to go through the re-fill process, something that we have done on a number of occasions without any issues.   As I say, a prescribed process of filling, starting the engine with the rad cap off, then adding coolant until the rad is full, before replacing the cap.  (no "burping" procedure or equipment required.   Easy....!😱

This time, as we adding coolant the radiator suddenly shot a fountain of water out all over the right hand wing, engine etc!   We were not amused!😡

But, we let it settle down, and I did squeeze the top hose a few times in the hope of clearing a possible air lock, then re-started the engine and continued as normal, with no issues.

MAybe you were just unlucky this time?