I can't find an answer to this on the wiki or in the forum old threads: does the temperature sender use a gasket? I ask because over the years I've gotten two different aftermarket temperature senders that go to the (weird, multiple gauges in one dash space) meter, and both of them fit fine and the nut/bolt thing screws in just fine, but the temperature sensor is just rattling around loose in there and needs a 3mm thick spacer between it and the nut that screws into the head, and that indicates to me that this may be a standard size but needs a gasket.

The Datsun 1200 temperature sender uses a taper seat for sealing

Nissan 25080-89900 GAUGE-TEMP ENGINE UNIT

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It might something on the threads that is not allowing the nut to screw all the way in. Perhaps some corrosion or some old "pipe thread tape" somebody used in the last 50 years, not realizing these are regular threads. Try running a blind tap down the hole to clean up the threads

oh and to answer your question, no there is no gasket used on the temperature sender