Dash light bulb cover/diffusers

They're just to tint the light greenish; go ahead and remove 'em if you like. I often do. There is one good option for brighter bayonet-base bulbs to fit the dash bulb holders used through '65. It is this one. The stock bulbs produce 25 lumens; these upgrades produce 85 lumens. Wattage is higher (6 vs. 4) but I have never had (or seen) a problem using these bulbs -- though if you run them at full intensity for long enough, in theory you could begin to soften the blue-green caps, so you might want to remove those.

The "LED bulbs" you'll find all over the net would not be a good choice. Their light output is highly directional, so your dash illumination would be spotty with bright areas and dark areas. The reason the gauge cluster housing is painted white inside is because the light radiating in a spherical pattern from the bulb needs to bounce around behind the gauges to light everything up evenly.

For the '66-up wedge-base bulb holders (which you can install in your '65 cluster if you like) there are lots more bulb options. There's a wedge-base version of the linked 85-lumen bulb, there are other clear-glass bulbs in numerous different output levels, there are good amber bulbs, etc.