Our 3/8" Ultralites are a direct replacement for both the Gibson and Epiphone Thunderbird bass. You will need Mini Clover or Y key tuners because the full size Clover and Lollipop keys will touch each other as they turn due to the close spacing of the headstock.
The 3/8" Licensed Ultralite in Y key is also a direct retrofit for the Gibson and Epiphone Thunderbird.
For more information from folks who have installed Ultralites on Thunderbird basses, check out this TalkBass thread.