I would love to buy a pair of these and also a pair of LED, Halo Headlights.

 

Cut tailgate lights open, drill the holes, mount LEDs, do the wiring, glue everything together, enjoy.

 

LEDs can be dimmed with fairly cheap logic boards. It would make it easier to match up to existing than trying to get the exact wavelength. There are some fairly cheap micro-controllers that will do this that would even let you do some additional interesting features if you wanted; ie sequential turn signal like the new Mustangs.

If there is enough interest I could run the math and work up a diagram if anyone has a wiring diagram for the existing tail/turn/brake/reverse lights.

 

Yes but it is more about color than brightness. Or you mean installing RGB LEDs?

 

Brightness and wavelength are completely unrelated. Making it less bright won't change the fact that you have a different colour red than the stock setup.
As per Vadimus, yeah you could possibly simulate it with trial and error of an RGB mix, but...
Throw in some 6803 embedded RGBs and you could use them to throw an animated finger at tailgaters...