I wouldn't worry about it boiling when the gauge is at 210. Car gauges are usually not high precision, so the numbers can be way off from actual temp.
If this an idle situation only you need to see if coolant is moving thru the radiator at idle. Start with a cold or cool engine. Lower the level 1” in the radiator and leave the cap off. Start the engine and let it idle in drive if an automatic periodically checking the thermostat housing with a temp gun. When you are positive the thermostat is open, is there any movement of coolant? If not slightly raise the idle maybe 100+ rpm, if there is now coolant movement you need to increase the speed of the water pump with a smaller pulley or larger on the crankshaft. If you have movement at idle you need a bigger or more efficient radiator. Not sure on a SBC but on YBlock Ford it’s a simple fix that works everytime…
I don't know what you guys we taught, but on my old cars, I always filled the rad to the top. Late model junk with an overflow bottle, the rad is filled to the top, and the bottle filled to the indicator mark.
Every cooling system must have expansion space . Old stuff puked on the ground ( low pressure systems) & nobody much noticed .
And you left the radiator about an 1” low. When I bought my 56 Ford with a #7 cap I filled it up because I forgot.. I didn’t put a puke tank on for 2-3 years so it ran low until that time.
Since your vehicle started life with an inline 6 cylinder engine I thought I might ask how far is the fan from the radiator? Maybe a spacer would solve the problem. Charlie Stephens
I fabricated an shroud . At idle the fan really pulls the air through the radiator. At 85deg day ,runs at about 180 degs. Trucke