There is a ton of junk on the market, so be picky; get the good ones and don't faff around with off-brand trinkets. Worthy bulbs: This is the warm-white one; this is the cold-white one; this is the yellow-amber one (I prefer these for general dashboard illumination with max legibility with min glare; they also work well in green-lens pilot lights) and this is the red one (for red warning lights). Relatively expensive compared to stock filament bulbs or cheap generic LEDs, but you'll never have to replace them again and your dash light rheostat, wires, circuit board, etc will run nice and cool.
The only "gotchya" points are minor: LED means Light Emitting Diode. A diode passes current in only one direction, not both like a filament. So if your car has a single green pilot light that flashes no matter whether you're signalling for a right or left turn, don't put an LED in it. And for the rest of the dash lights, it matters which way round the bulb is installed in the socket (or the socket in the circuit board). If the LED is installed backwards, it won't light when it should. If there are other LEDs in the system (as there are in the dashboard) then just one backwards LED can create weird new paths for electric current that shouldn't be there, causing a no-light or no-light-when-supposed-to or light-when-not-supposed-to situation.
So, fashion a jumper to ground the cluster properly (when installed in the dashboard it grounds via dashboard metal). Install one LED bulb/socket assembly. Try it out to see if it works correctly. If it doesn't, remove it, turn the bulb or the socket (not both!) 180°, and reinstall it so it works. Then move on to the next one: install one more LED bulb/socket assembly and try it out. Proceed this way until all the bulbs are installed and working.
Note the links provided here are for wedge-base bulbs as used in '66-up cars. Legitimate bayonet-base LED bulbs are much scarcer and more costly; easy workaround is to install the '66-up dash bulb sockets for the wedge-base bulbs. They fit right in place of the earlier sockets. That's these (sold individually, one "each" at the listed price, even tho the pic shows two—hence why two buyers left upset reviews, thinking one money got them two sockets).
Beyond that:
• For brighter gauges you can remove the blue-green plastic balloons over the gauge illumination bulbs in your cluster; pick your light colour by choice of bulb.
• Make sure your headlight switch rheostat contact and coil are nice and clean; if yours is crudded up or worn out, get a new headlamp switch—good American-made ones come from Old Car Parts Northwest.
• While you have the dash apart, put some 70% rubbing alcohol on a cotton ball and swab the gauge faces to clean away the age-related yellowing. Maybe fetch a small paintbrush and a little bottle of fluorescent red (or green or orange or whatever you prefer) paint and repaint the gauge needles. I stick a Post-It note to the gauge face behind the needle to avoid paint oversplash, then just remove the Post-It once I'm done. I used fluorescent red on the gauges in D'Valiant (pics on page 4 of the linked writeup), and I used fluorescent yellow when I did this on What The Truck and it turned out great:
