Rear brakes lock up, front factory disc 67 Barracuda

Alright, so I installed the 37696 (13/16") wheel cylinders onto the rear brake assemblies and it did help considerably but they still do lock up. Now having said that, I looked back at Dans comments and realized I should have used the smaller 3/4" cylinders because I do have the 7-1/4" rear end.... duh. All is not lost though, I'll buy the proper ones today and use the 13/16" slave cylinders on the ragtop Cuda which does have an 8-3/4" rear end.