The original 7 1/4 rear in my 65 Barracuda, Commando 273 and 4-speed lasted 90,000 miles. The junkyard replacement I got for it lasted maybe 25,000 miles. Then I switched to an 8 3/4 rear.
I have an A body 8 3/4 rear to put in my 63 Valiant slant six if the 7 1/4 in it now ever breaks. So far it hasn't.