I can't seem to find bright LED's with a screw base to fit the notoriously dim Smith's Speedo/Tach/Boost gauges, so I set out to modify available stock. You can get 5 x 5050 SMD LED E10 screw base bulbs, and they are brighter than the OEM filament bulbs, but there's only one in each gauge.The Smiths on my '85 have green plastic domes inside the gauge that limit the amount the bulbs can extend inside, so try to keep the bulb as short and round as you can. Color change is not an option here!
I wanted to move up to the newer, brighter 5730 chips in a screw base to work with a dimmer. I started with some 8 x 5730 SMD LED wedge bulbs.
2pcs T10 194 W5W 5630 5730 8 LED SMD White Car Side Wedge Light Lamp Bulb | eBay
You'll need some sacrificial bulbs to get some spare bases to work with. Avoid the aluminum ones as they are hard to solder. Crush or twist out the old bulb from the base shell after sucking the little blob of solder off the tip. Add a little solder to one side of the inside of the cleaned out base to ease soldering the negative side later.
Check the wedge style bulb with 12V and mark the positive side. Grind the sides just a bit so it will fit within the base, then solder a stiff wire to the positive contacts and bend it so it will exit on the centerline. Add a generous blob of solder to the top of the negative side for later attachment to the shell.
You can slip a little shrink tube on to keep the positive wire from contacting the shell, then just slip the unit into the shell with the positive wire exiting the bottom of the shell and the solder inside the shell lining up with the blob you put on the negative contact. A little heat on the outside of the shell should solder the negative side to your shell. Add a blob to the bottom wire and you're done! If you want you can drip a little clear epoxy in the base to add strength.
As with any LED's your original dimmer will have limited effect- but that's another topic.