You are right, the speedo and tachometer have bulbs of their own (perhaps also the clock, I forget) but the little gauges are lit by bulbs that are mounted beside them and the light leaks in via slots in the instrument panel to the little slots on the side. A little white or silver paint applied to the slots in the instrument panel might help it on its way
Led bulbs definitely improve things. I used
https://www.bettercarlighting.co.uk/index.php
Led bulbs normally require a different rheostat as they draw so little current, but I left the old green incandescent bulbs that light the centre dashboard controls and switches in situ and found that the existing dimmer rheostat worked fine as they continue to draw sufficient current to drag the voltage down, dimming the LEDs at the same time.
While you’re at it make sure that the ammeter connections are tight, copper creeps over time and they may do with tightening up.
LEDs in interior lights make a world of difference.
Go for it