Yes they did (on some).
I will see if I can dig up an older post on this board and the Roadrunner Nest where it was discussed in length. There was a change over later in the year where the factory started to use seperate headpipes. I owned a '71 U-code GTX that still had it's original H-pipe with the factory flattened areas. It had a SPD of Dec 17th 1970. A few other members have survivor cars that still have the OEM H-pipes. We compared our Broadcast sheets and the cars with H-pipes listed this in the front pipe boxes:
EXHAUST
LT RT
00 01
The cars equipped with regular headpipes had:
EXHAUST
LT RT
93 76