[spacedebris]

I've never fooled around with the vintage gauges personally. However I did have the same kind of problem with my 89 GMC Jimmy with the temp gauge reading over 240 when the temp gun indicated only 205. In my case it was a bad resistor on my gauge. At the time I was in school for electronic engineering and instead of just buying a new gauge, I replaced the resistor (cost me 95 cents from radio shack), then checked again. gauge read between 205-210 when the heat gun said 205.

Now granted this was with a system over 2 decades later but the older gauges are likely built in the same way, sender sends a signal to the gauge and the reading across the resistors determines how high the needle goes. Since you said you replaced the sending unit, this would be my best guess.