Changing the rear quarters on my Dart conv. from a 69 to a 70

Sounds like you have a plan, but if it was me, I would add the SWINGER/flower emblem under the rear-side windows (one each side). If you cut and replace the rear section just before the rear side marker, it will hid any imperfections of the repair, under the swinger stripe. I could be wrong, but I think the GT emblem was installed on the front fenders, next to the front doors.
Mine came with the Dart GT emblems below the quarter windows behind the doors. I'm reasonably sure that when I do the splice that the repair will not be super noticeable plus I will cover that area with the same type of spray on coating as the factory did in the trunk.