The '67 lens will bolt into a '68 housing....however, the '67 lens takes 5 screws and the '68 lens uses 4....so when you install a '67 lens in a '68 housing, you would only be using 4 screws instead of five.
I failed to mention that the new hole would also need to be cut in the back of the lens. It needs to be about 10% larger than the diameter of the bulbs globe too. I want to say I used a 1&3/8 hole saw, memory fails me.