If the mirror is seriously misaligned I would expect that the viewfinder image would show some vignetting at top or bottom, though I believe the viewfinder coverage is not 100%. If the camera is the F-1n (not New F-1, AKA F-1N) then the meter will not work with the screen upside down. I have looked at both cameras and it seems it might be possible to install the screen upside down.