I tried to come up with a custom inner C to use those TTB 50 knuckles to build custom front axles, but the Super Duty axles became so prevalent that it just wasn't worth it.
Not 100% interchangeable from the spindles out. 89 D60 should be kingpin knuckles with a separate caliper bracket that bolts between the spindle and knuckle - the TTB 50 spindle is not long enough to go through the caliper bracket and still engage the knuckle (that would leave only the 5 spindle bolts to take all the load). Other than that though, the 50 hubs themselves should work fine on your existing 60 spindles. Just don't use the 50 spindles.