Oh, I see you're thinking I mean front end! No, rear end assemblies interchange 60-72. Front ends can use crossmembers as late '87, I believe, provided you drill some extra holes. The only reason I can think of to stay under '72 is to keep 6 lugs. Otherwise I would use the late stuff just cause it's less apt to be worn out!
Btw the rear end bracket kit is at:
http://www.truckandcarshop.com/ in the 60-66 catalog, P/N: 60-146131, page no. 113
