Got a used 7140 rt axle shaft to try on the 5488. Side by side, same length, outer yoke splined end same. Slid into axle, looks good. Will know for sure next week when the rest of the parts come in, but looks like it will work.
If you just need an outer yoke, looks like you could use one off a Magnum. U joint cups are same od, you would just need 2 of the external snap rings. ( 88 used internal snap rings, Magnum used external )