Tractorboy;
A viable alternative to mounting the electrical sensor on the head is to try to find a thermostat housing that has the larger threaded opening on the side below the thermostat. This is what I have on my 261 with the electrical sensor. Regarding the resistance, most newer sensors you can find at NAPA etal, just don't seem to work with the older gauges. I've tried a ton of inline resistance options for matching my gauge to my sensor but with no luck so far. (The sensor is a negative coefficient type where the resistance rises as the temperature cools and vice versa) Have found a used one which is now on my 261 but haven't started it up since the rebuild to check to see if it reads correctly. If I did this pic right, you can see what I mean.

http://rides.webshots.com/photo/2869036240010188466uuTHjC

Country Roads, West (By God) Virginia
http://community.webshots.com/user/geezer