The "gap" between the dutchman and the trunk lid has always been a sore point on the 2nd gen chargers, I have NOS qtrs and other NOS/original sheetmetal on the rear half of my 70 Charger, and I was dealing with a 1/2" or better gap at the dutchman/trunk area, my solution was to eliminate the end cap plastic gaskets, grind/sand the face of the caps, becareful with that, but the biggest place to remove material is the mating edge of the cap, becareful of the lower tang that rolls into the bumper well, use a course file to carve down the front mating edge, I had to slightly taper the side profile so that I could "tip" in the top edge that lines up with the trunk edge, worked out well just manipulating the end caps, allowed me to close up the front dutchman/trunk gap to just under 3/8, which was a LOT better than the "Great Divide" that was present before, and no one even notices the end caps are slightly thinner in thickness
Mike