Actually, I thought the 68 Shelby just used the standard Mustang bucket. After all, doesn't the headlight sit right where Ford put it?
Now the 68's had only 2 headlights and 2 foglights, those required the larger headlights.
Now, in 67, Shelby used 4 headlights. Federal law required that these be the smaller sized ones. AND, the nose of the 67 Shelby extends out some 4" forward of where the stock 67 Mustang lights were. The outer headlight buckets are out of the parts bin from Ford, maybe the truck line with custom mounting. I'm not sure.
The high beams on a 67, well, there are 2 ways of mounting the inboard lights, known as early and late. The outboard highbeams were all the same type of mounting.