The contact paper really doesn't match the original woodgrain in texture and finish. The contact paper is extremely smooth and somewhat glossy compared to the original that has a "texture" and no/very little gloss. The pattern and color are good though in my opinion.
I think that to get it to adhere well and look good, you'd have to sand the pebble grain smooth. Getting it to conform around the curves of the ashtray might be difficult to do without getting wrinkles/creases and keeping the "grain" straight. Also, you should probably cover the entire dash insert so that it matches-and possibly the floor console if you have one. I've seen it done and it doesn't look too bad, but not "stock"-it just depends on what you want.
PS, I might be wrong, but I think early 68's with the woodgrain still had a pebble grain ashtray-it wasn't available till later in the production run. So using it could still look "correct"
'68 Camaro base coupe
'92 Caprice wagon