6v LEDs bulbs with the right bases seem to be getting more common.
Also, positive/negative ground 6v LEDs are now available.
The "problem" with the LED "bulb" is sometimes the orientation of the individual LEDs relative to the truck-lamp's lens.
Cosmo points out the need to switch from the old style flasher unit to a newer 6v electronic flasher. This is because the old style flasher "flashes" based on current draw - the LEDs draw very little current.
Here is side-by-side LED bulb and normal bulb on Bill Miles' 53 GMC.
