When I was restoring my 1971 GTX way back in 1988. I ordered a ACC carpet set from Legendary and it came in with the 70 and earlier retangular heel pad.
I sent it back because the original heel pad that came in the car was the E-body style one. That is the one with the cutout notch in the upper right corner.
My understanding is that the 1971 and later B-bodies had the E-body style heel pads.