I'd just get same type incandescent bulbs that it started out with. They aren't expensive and lasted 25 years.
As far as how hard the LED ones are driven, if they dim properly (linearly enough compared to other lights controlled by the dimmer) then just don't run them at full brightness. I never have my dash at full brightness, just reduces my night vision.
One other factor with the LED bulbs is that (especially the cheap ones made for multiple purposes besides automotive) they are typically designed to run at 12V, with a resistor in series. Doing some math...
Suppose a 3.2Vf drop across the LED, and the target current is 73mA (a value I chose, just because it would use a very common resistor value of 120 ohms, and also that I have T10 landscape bulbs that I switched to LED and they do consume about 73mA) but the math works similarly with some other target current), that makes the LED power 0.073A * 3.2V = 0.23W
Now suppose same LED and resistor but running at approx 14.4V that the alternator provides, and it's running at 93mA or 0.30W.
If it's a multi-chip LED bulb with the LED dies in series, the math could work differently for the worse, like suppose it has 3 LEDs in series for a 9.6V forward drop and 25mA current through the series, 96 ohm resistor and 0.24W drive. At 14.4V the same 96 ohm resistor causes a doubling, 50mA current and 0.48W drive.
I faced the same issue with the 73mA LED bulbs that I swapped into my landscape lighting. It uses an unregulated 12V transformer, and the load from the incan bulbs pulled it down to not much more than 12V, but with the far lower load of the LED bulbs, voltage floated up a couple volts and they didn't last a single season, so I put a buck regulator circuit in series to get the voltage back down to 12V. That seems impractical for adding to a vehicle lighting circuit, as long as the plain old incan bulbs exist at low cost. Your call.
