Have you searched eBay for the out of production temperature sensors?
The 10 ohm resistor was added to lower the reading of the gauge, not to make it more accurate. Seems owners were not used to an actual temperature gauge, as opposed to an idiot light, and they unnecessarily were concerned and taking their cars in to fix the “problem”.
so the technician at the dealership would add the resistor and with the now lower gauge reading, the owner was happy and all was good. I think the philosophy behind the 260° gauge was to once again placate the owners, as the needle was now not nearly as close to the end of the range.
if you truly need to have an accurate reading, you need an aftermarket gauge and it’s matched sensor. If you want to keep the original look, there are Speedometer/gauge specialists who can place modern mechanisms in old housings with the old face plate.
at the end of it all, the temperature gauge just provides a number. As long as a Pantera is not boiling over when parked or audibly boiling water in the heads, there are no cooling issues.
An inaccurate temperature gauge is not useless. Once you become familiar with its normal readings under normal conditions, observing something out of the ordinary would be your cue that the cooling system needs a close inspection.
Larry