[hatzie]

There are several temp senders used between 73 & 91.
78 is right on the edge of the end of the obsolete 1/2 NPT Nailhead sensor. 1/2 NPT is tapered 1/2" x 14tpi.

Some 79-91 motors used 1/2NPT and some used 3/8NPT. The 79 & later engines with a 1/2NPT sender have a 1/4" disconnect tang rather than the older nailhead connection and the thermistor resistance curve is different from the older sensors. There are at least two 3/8NPT sensors. One with a simple 1/4" tang and one with a sealed Metripak 150 connector.

The LS motors use an M12x1.5 metric thread for the sensor. You can buy a 3/8NPT to M12x1.5 adapter for LS swaps.

You have to match the temp gauge to the temp sender or it will not read as accurately as they were originally setup to read. Different may not be that bad. Honestly these gauges are a matter of getting used to the "normal" reading rather than any real accuracy. As long as the gauge reads near the middle when the engine temp is somewhere between 190°F and 210°F it'll work fine. I'd try the 1979-1980 3/8NPT sender in an LS adapter and see if that makes you happy.
If you just can't stand having a slight mis-match I'd get a temp gauge from a 1979-1980 to go along with the sender. The fonts and colors should match 1978.