I saw LED taillights at a Pilot gas station.
I did not get to measure them. They are described as 4" diameter and fit the standard heavy truck rubber grommets. I plan to see if they will fit the taillight housings on my 1979 SS.
These are really coming down in price. They are on special for $17.99. Only a few months ago they were $40 each.
The 2" and 2-1/2" diameter red and amber marker lights are now 4/$19.99.
LEDs are reputed to be very long lived - 10,000 and 100,000 hours are common claims. They also draw only a tiny fraction of the current of incandescent bulbs, reducing the draw on the tow vehicle taillight circuits to very little, which may be important on some late model tow vehicles.