Dwaine,
Back in 1965, the sheet metal panels were built and welded by humans. There will be some differences from car to car, but for the most part, the taillight housings should fit the same profile as the body's sheet metal. The factory would fill those sheet metal seams with lead before painting. To compensate for any minor differences, especially at the corners, a 1/2 thick, soft rubber gaskets is used. I would use the taillight housing as a guide to get the sheet metal as close as possible to the same curvature. The new, thick, soft rubber gasket will seal the deal. Be careful while bolting the center emblem trim in between the two taillight housings so as not to break those two tabs off. If the tab starts to bend while tightening the nuts, stop! as the next thing to happen will be that the tabs break off. (voice of experience) Just take your time and everything will go well.
Good luck,
John