I had the same problem with both the panel gaps on the trunk and the hood, someone years ago had fitted new wings/fenders and had not paid sufficient attention to the gaps, my solution (which may sound extreme but worked) was to run a length of masking tape along the edge of the trunk and hood at a distance that is slightly smaller than the gap that you require, I then used my grinder and took off enough metal to just touch the tape (the gap should still be too tight), using a large hand file I took off the remaining metal until I was happy with the gap, a little time consuming but you get the result that you want.
I had previously stripped all the paint off so I could see the spot welds.