Just a quick follow up on this.
Looking at the Hamtramck color and trim book pages, it appears that the bucket seat with a buddy seat option for 4 doors didn't appear until '75. This would imply that the DS seat would have the shorter track on the inside due to the catalytic converter hump that was added at that time (I think). I don't remember needing to do anything to my seats but swap to better rollers so maybe it was a later option in '74 as well?