I had a very similar problem on my 69, it was a pain in the @$$ to find. My front right corner was sitting about 3/4 higher than the left. I noticed it after I did a complete front suspension rebuild including springs. I could see the difference in the fender gaps. When I took it for alignment I had them check the frame and suspension. Everything was right where it was supposed to be. No crooked springs or bent frames. So after another week of poking at the car, checking body panels etc, it became apparent where the problem was. My left rear spring was weak and caused that corner to dip a ever so slightly, thus bringing up the front right. Solution: New rear leafs. I already had them, but hadn't installed them yet. Just something to check.
1969 Z28, 406SB