Sponsored Links (Register now to hide all advertisements)
If this were my car, and after noticing that a LOT of tail lights are pointing uP, I would assemble the light to the bracket and with a level, plumb the assembly on the fenders to center them on the fender, and bubble them to the vertical, mark holes and repeat for the other side.
This isn't really as brain surgery as you are thinking it should be unless you are building a "Fine Point" restoration, and even then, I doubt a judge carries a tape measure for exact dimensions of tail light mounting.