My '69 Dart GTS 440 M-code vinyl roof has been removed but on one side it is still visible where the "seam" was located. The width of the GTS's roof at the front of the door up from the point of the vent window is 50" across. The visible "seam" line is 6 inches inboard of the edge of the roof on both sides or 12".

50" - 12" = 38" on my '69 GTS. If the "center deck" is the space between the two "seams" then mine is 38". Hope this helps.