Thanks for everybody's help here. One more question:
Where does your temp gauge normally sit once the engine is warmed up - say a 20 minute drive in 75 degree weather, in town driving but without much stop and start?
I purchased several senders including some recommended here for my '79 crate engine with 3/8" threaded sender, and all put out too much resistance to work with the 100 ohm gauge resistor. I end up with too much resistance and the needle stayed at the cold end.
As a temp fix, last night I replaced the 100 ohm resistor with a
variable resistor (ie. a guitar volume pot I had laying around).
I did this and dialed in a lower amount of resistance, and the temp needle now responds appropriately - reads cold when the engine is first started, and warmer as the truck warms up until it comes to rest close to the middle. I will likely borrow & use an infrared temp reader to see what the actual temp is, and see how the gauge matches it. As a longer term fix I may have one of the 1/2" threaded senders turned down to see if that works. But for now...
Pic on the left is right after I started it, pic on the right is after a 20 minute drive:
