The issue of the correct level of voltage getting to various interior light bulbs is important. When the cars were new, the interior and cluster lighting was "normal", with the map light in our '66 Newport being bright enough to read a map at night with no issues.
With time and use, dust can migrate into areas of the instrument cluster and the reflectors of other interior lights to dim their effective output by a noticeable amount. Another issue is the condition of the reflectors! When new, they were shiny and "chrome-like", if not chromed. But 50+ years later, much dust migration can happen, by observation. Especially in areas where it is uncommon for vehicle to have had factory-installed air conditioning, where "windows down, cowl vents open" was more common most of the year.
Combine this with dust also settling in on the insides of the related lenses and less actual light output. NOT to forget that some of the lenses have yellowed a bit with age and heat.
SO . . . cleaning and refurbishing the reflector area of the lamp, plus wiping any dust off of the bulb and inside of the lens are first steps. Then comes the voltage checks, if needed, which can possibly result in an investigation of the wiring harness related to the lights themselves. Then the rheostat in the headlight switch itself. Which can then lead to the bulkhead connector's terminals. Which can then lead to the vehicle charing system.
In most cases, things on the output side of the headlight dimmer can make things better. Just as making sure the charging system (alternator, battery, and related cables) are doing what they are supposed to can be the next obvious checks. IF a prior owner or similar has made any modifications to the harness for added equipment, THAT can become part of the mix too.
It seems to be "trendy" now to automatically perceive that adding LED bulbs into the mix is a quick fix, which it can be, BUT if there are other issues at play, they can be an expensive-to-me Band-Aid fix. LEDs can also be an indication of other vision-related emerging issues, from my own experiences. Reasons why older people are usually at home after sundown, other than for potential personal safety issues. Whether in the 1970s or now.
End result, having instrument panel lights that are "too bright" can decrease the ability to see down the road, in the fringe areas of a vehicle's headlight beams in the process. What is more important, seeing the speedometer at full LED brightness or that deer "in the shadows" in the bar ditch that will be frightened by your vehicle, as you're driving 60+mph on a dark highway? Personally, that incandescent-lit instrument cluster at 70% max brightness can be the best option, to me. PLUS making sure the headlight bulb lenses are ALSO clean!
Just some thoughts and experiences,
CBODY67