I've been working on a 68 to 69 tail light conversion on my 68 GT.
I never did care for the extra large tail lights on the 68.
Made a prototype sheetmetal piece to get close as possible for the 69 mounting.
Found the best location for it and held it temporarily with sheet-metal screws.
Mounted the 69 fender extension to check for fit.
Lens fits pretty good. I will fill in all the holes with muggy weld to delete the chrome trim.
I will modify this one and then make two final mounts.