I think I know a VDO temperature sender that should work reasonably well. I did some research for Dan Richardson to find temperature senders that were 1/8 NPT. One sender I tested was the VDO 323-095. By tested, I mean I heated up a pot of water across various temperatures and measured the resistance of the sender. My conclusions about the VDO 323-095 were:

The VDO sender resistances are lower than the Temp gauge expects. That means the temp gauge will read higher than the actual coolant temperature. The VDO sender will result in the Temp gauge reading about 9 degrees higher than the actual coolant temperature at around normal operating temperature. E.g. When the coolant is 185 degrees, the temp gauge will read around 193 degrees.

I also had some thoughts on adding a resistor in series to get the temperature gauge readings more accurate within the normal operating temperature range, but maybe it is sufficient to know the temperature value is always off by a little bit.

I checked the VDO web site and they are indicating the VDO 323-801-001-007N is 3/8 NPT and it has the same resistance range values as the 323-095 sender that I tested. I expect the only difference is the size – 1/8 vs 3/8 NPT. It appears that the VDO 323-801-001-007N part number is also VDO part number 323-094.