There is another difference.. The 67 doors have a rectangle opening for a special fasteber that catch the screws through the base at an angle. 68 up doors have drilled holes in the sheet metal.
This difference may cause the later bases to tighten up to the door higher or lower than the 67 did.
If rear armrest bases are part of your plan and your 67 didn't come off the line with those, Y ou will find the rextangle holes in the metal but no clips installed. That's all I can offer. Hope it helps