You basically have a 60"x24" Tee over 2" thick. You should not expect too much "give" in this connection although the stiffness shown is < the 1E12. default CII stiffness. Klong of 5.5E6 Nm/deg (48.E6 in-lb/deg) seems low for this connection, but I have not been runnig any FEA in a while. I would try at least one alternate method to compare the sitff from WRC-297.