I thought I'd follow up on this thread after "fixing" the issue. The best reason I can give as to why the temp gauge now works is that I touched everything. I had replacement sensors and gauges fitted, then went back to my original pieces and voila! Probably I had missed a cruddy connection somewhere and restored a ground contact, even after taking care to measure things. In the course of doing all this, the oil pressure gauge also now reads correctly, something I was going to tackle subsequently.
I have two pieces of info to leave here, followed by a few musings:
First is that the two temperature sensors I have measured from 7-10k ohms at 50*, and about 1k ohm at boiling. I hope this data helps someone now or in the future.
Second is that the internals of the gauge are exceedingly simple. There are two coils in series and a simple needle movement. I didn't measure the resistance across the meter. Best to follow the TM and confirm/deny the existence of the voltage drop.
I bought a NOS meter for a replacement, thinking that the metal gauges are more robust. Turned out the NOS gauge was BIB (broken in box). I was dubious it was going to work because it had zero resistance across the contacts and did not drop the voltage to the sender as the TM instructed. So I installed it and confirmed it was kaput at the get-go. There was a very fine wire connection inside that is broken and will be difficult to patch.
My deuce really is cold blooded. I can block the radiator completely, drive it around (unloaded) on surface streets in 60* weather, and it won't broach 170* operating temperature. I think it's time to check for the existence of a missing or stuck open thermostat. Onward and upwards.
Rick