Does anyone know for sure if the wheel opening moldings on the rear wheels for 67 and 68 RS models are different than Standard models, and if so how?

I don't know for certain but I believe they are the same.

Standard would not have any bright trim at all (except the spear down the side at the botom of the doors). Option code Z21 "bright trim" includes wheel well moldings and bright trim around the rain gutters. Option code Z22 "Rally Sport" adds bright trim panels to the fender, door and front quarter panel on top of the Z21 moldings.

Only one kind of rear wheel well moldings each year, but 67 and 68 are different due to different lower body moldings on each year.