Like many before me, the correct speedo cable clip is a difficult part to find. Some cars have 2 holes in the tunnel and use 2 clips in the tunnel area, while others have one hole in the tunnel. In addition, there is a hole behind the ignition coil on all cars for another clip, close to the cable's grommet, just before where the cable enters inside the car.
One restoration parts supplier offers two versions of this clip p/n 3772324, one of which is described as "assembly line correct" at a higher price. I've tried installing this "assembly line correct" clip and find that one of the short locking fingers breaks off when trying to get it into the sheet metal hole and/or when expanding the locking fingers. This happened with two of these clips. Reviewing posts from others on these clips, it is apparent no one is making a workable version of it.
In the meantime, I've found another style clip that does a great job of supporting the speedo cable, in any of the 2 or 3 locations for a speedo clip and securely fits into the existing approx 5/16" drilled hole in the sheet metal. The clip I found is actually a MoPar brake line clip for use with armored brake tubing (measures approx. 5/16" dia. over the armor/spring material). It was used extensively on 1960's and early 70's MoPar cars. This clip not only fits the existing sheet metal hole nicely, but also gives a secure fit to the Camaro's speedo cable.
Until such time as I can find the correct style Chevy clip that actually installs, expands the locking fingers properly without them breaking off, and stays put, this MoPar clip is going to be used to keep the speedo cable properly routed. They are available from any of the MoPar restoration parts suppliers.