I don't have an answer but I can tell you mine looks almost identical including the driveshaft rpm. Its a 727 3450 lbs runs 9.60's. Have not had time to investigate it - don't know if it started with the new carb or not. Have not looked at my old data yet.

67 Coronet 500 9.610 @ 139.20 mph
67 Coronet 500 (street car) 14.82 @ 94 mph
69 GTX (clone) - build in progress......