[Dayton601]

Could you take a few more pictures, , step back a little so can see the whole area around tail light opening, trying to picture what spot welds you cut, ( drilled) My second thought is that the metal needs shrunk a little, most likely in the flat panel area, that or heated a little when you have the board pushing with pressure, something is pushing back hard for it not to stay in place with out the board, I'm also trying to vision how you welded the bottom of the panel to the inside trunk pan at the very bottom, that donor piece you welded back in could be just a little bit different shape from the old piece, but I can tell you, some where there is pressure and it needs cut or stretched, if you cut and there is a gap you can always tack in a small strip to fill it,

My third thought would be to jack it further past were the gap would be acceptable thus stretching the metal some where, and when it comes back after removing the board the gap may be close, only problem is for every action there is a reaction, and if your avoiding trying to not have to go back over your body work, well you just might have to o