8 1/4 rear question

Will a 8 1/4 rear fit in a 71 duster or did they come stock with them? Have a 7 1/4 in it now but want to go to big bolt pattern. What are my options? Can u change out axles? Have to go to the 8 3/4 rear? Would like to just put in a 8 1/4 in it? What is length of it drum to drum?