Muu Muu
1) Disassemble what you have done till the spindle can be moved to the workbench.
2) See if a file will clean up the threads so that you can spin the nut either by hand or with a small amount of resistance. If not replace the LBJ.
3) After you have it working or have replaced the LBJ, install the LBJ through the LCA. Make sure the slot for the cotter pin is facing front to back.
4) Put a small floor jack under the spindle so the taper is seated in the LCA and tighten it down and install the cotter pin. Access with a wrench and a rachet will be 10x better.
5) Install the UCA down over the spindle. Again, make sure the cotter pin slot goes front to rear and tighten.
Aligning the cotter pin slots seems like extra work but if you ever have to fool with it again you'll be glad you did it.