to install 68 tail lihts in a 67 you have to modify the sheetmetal. The 67 buckets at shallow and the 68/69 are deeper due to the back up lights in the tail light
I have seen 68 & 69 grills in a 67, was told that they were different the way they mount, so not sure how difficult to do
I do not believe the cars are different in lenght. The 67 never had the chrome exhaust tips and due to the offset gas tank and single strap they only used dual exhaust on the 383 cars
68 they changed to a dual strap on the gas tank that run front to back instead of frame rail to frame rail and my guess is to make it easier to install duals
It can be done, how much of a 68 do you want? If you really want a 68, sell the 67 and buy a 68 might be easier, unless you only want the exterior stuff.
The radio antenna is also different on a 67. The fender mounting hole in smaller on a 67 ( found out the hard way after the car was painted)
I was going to put a 69 grill and tail lights in my 67, but after the difficult time getting the body guy to do something, I gave up on it.
I had a 67 hardtop that I was going to clone into a 69, but decided I'll find a 69, so I sold the hardtop