There are a few Ford 9" and 8.8" rear ends that come in 59", 60" or 61". Tougher too. The 1973,4 ,5 Mark V and Continental town car have a disk brake 9" rear axle, it's 63" wide and they are cheap. It also already has 5 on 5 bolt pattern like your truck.
Everyone can shorten a 9". Or buy...