The symmetry on custom work is always the biggest pain, but it's all worth it in the end. Someday I'll actually measure out my stuff and catch stuff like that before I get too far along!
The lack of good reference for this one is the biggest stumbling block for me, but at least I have one straight on side shot to work with. A nice large, clear rear shot would be ideal so I can see exactly what the recessed tail lights look like, but I'll do what I can with what I have.
There's nothing worse than getting close to final paint and realizing something isn't symmetrical with the body. I made sure to tape both rocker panels to a piece of glass when I was cutting up and reassembling the body, so it should be square.