Even into the 66 model year the 7-1/4 was the most common to find behind ALL a-body drivetrains. My old 66 Commando 273 four speed Barracuda had a 7-1/4. After breaking an axle (the PO had broken the other axle AND two sets of spider gears) I installed a 3.23SG 8-3/4 from a 67 Dart (273 four barrel/auto) and never looked back.