Since the 62-72 B Body drum brake spindle is the same dimensionally as the 73-76 A Body disc spindle and takes the large ball joint, you should be able to use the B spindle with 73-76 A UCAs and run the wider 10 inch drum brakes (or even 11 inchers). Personally, if I wanted the 5 on 4.5, I'd just use the 73-76 A Body disc set-up.