The thread in the 216 head is a 5/8" bolt thread- - - -it's not tapered like a pipe thread. Most better-quality gauges come with various adapters to make their sensors compatible with that thread. The tech guy needs to come and talk to my garden- - - -he's shoveling "fertilizer" by the ton! Try this- - - -put the sensor into a pan of hot water, and compare the reading you get on the gauge to the water temp, measured with a laser gun. I think you'll find the reading to be somewhat different than the actual temperature of the water due to a resistance mismatch. (I'm assuming the gauge is electric, right?)

Using the block drain hole will create a really erroneous reading, since most of these old engines have a thick layer of crud at the bottom of the water jacket, and the drain port is as low as it can go so all the coolant gets drained. I'd suggest finding a flat spot on the cylinder head near the original temp sensor location and drilling and tapping a 1/4" NPT hole for the sending unit. You could also drill and tap the original temp bulb hole to 3/8" NPT. The engineer who designed the head 75 years ago or more is probably long dead- - - -he's not going to rise from the grave and haunt you for altering his design. With the sensor directly contacting the coolant, you can confirm whether or not the tech guy is lying.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!