Hi Kim,
Welcome.
When rebuilding my (66) driveshaft I had a c-clip / retaining ring issue as well. However, I couldn’t properly seat the retaining rings in the groove, no matter what I tried. After eventually measuring the thickness of the rings, I recognized that the new ones that came with the U-joints were slightly thicker. (Not much, I believe 0.2 mm or so). I ended up cleaning and using the old retaining rings.