Kurt....I'd like to use the 31 splines, but I was told the 31 spline axle has a larger bearing, and won't fit the '71 rear end. I guess I need to check into that a bit further. My 3rd member is coming out of a 1989 F-150, and that unit is about 5-6 inches too wide for my '48 F1, and the spring perches would have to be relocated as well. The '71 rear end that fits right in is a non-posi. I stiil haven't decided whether or not to do a leaf spring set-up, or a 4-link.