HI Jamie not sure if this is any help to you now as maybe you've already cut the tailight? but I replaced my tail light some time ago with a combined LED brake tailight, it fitted straight in and was brighter than the filament type bulb by a whole heap. Got it from a local shop that stocked only LED lights was a little expensive but certainly worth it. Only reason I did it was LEDs are supposedly more reliable and I really like the taillight and brake light to work especially at night when clowns can run into you.
Going to fit one to my old 79 Bonneville next, that will be the true test of its ability to withstand vibration....