The 60-61 main interior floors have a large hump to accommodate the cast iron automatics. in 62, the hump was reduced in size. The 62-64 floors will fit right into the 60-61, as long as you don't need the large hump for transmission clearance. 64 Chrysler interior floors and trunk floors will fit in 60 Plymouth's etc. The wheelbase difference is in the rear passenger footwell floorpans. The Chrysler rear footwell floors are 4" longer than the Plymouth floors and Dart-Seneca-Pioneer Dodges.
The trunk floors are all the same in all 60-64 full size Mopars. They did weld spare tire brackets and other brackets to the floor depending on year and model, but the trunk floor itself is identical.