The real answer isn’t entirely about the light bulbs. There’s a white/reflective ring inside each gauge housing, it’s job is to gather and disperse the light from the bulbs and provide a nice even cast of light across the gauge.

That ring has likely deteriorated and lost its reflective coat so now you are getting a fraction of the light.

A brighter bulb can provide a band-aid fix but if you want the gauges to look like they did in the 70’s you should plan on taking apart your gauges and cleaning things up. And recoating that ring.