The 'clips' are already on the adjuster screws when you buy them, if you buy the 'correct' screws. As shown in this photo from megapartsusa.com, the adjuster screws for 1968-1971 two headlight setups like '68 Barracuda have a white plastic (or nylon) part with a very thin metal cage around it that pops in and locks in the square hole in your fender.

(Looks like one of the plastic parts is backwards in this photo.)

Be careful: Once the caged plastic part clicks in, you ain't gonna get it out again without destroying it. That is, unless there is a special tool or secret trick I don't know about.

If in the photo, the three screws on the left are correct, you have to insert them from the rear of the square hole, and there is very little room back there. You'll need to remove the galvanized bucket that the headlight nests in. Or maybe even grille parts?

IMO, just about everything about the design and workings of the Barracuda make good, logical sense. EXCEPT these headlight adjusters. A crummy design, seems to me.