Muncie guys please help!
 

I have a 63 muncie m21, that I disassembled to replace 2nd gear... I have reassembled everything and am to the point where I am putting on the tail housing...
The issue im having is when I tighten down the tail housing bolts everything locks up... I loosen the tail housing and everything spins freely... I have checked to see that the reverse shift selector is properly in place and the speedo gear is properly aligned... I have taken it apart and put it together about 10 times following the steps precisely and every time the gearing locks up

What am I doing wrong and where do I start?