I think it is better to use the yield stress instead of the stress that you had found in the first part.
Try fs = 346MPa
at the top of the hole σref = 346/0.75= 461.3 MPa
σ_ref=6M/(w^2 t)
from this equation M = 6.8235496 KN.m
same for the top plate
K2 = 1
σref = 346 MPa
so M = 5.118 KN.m
I...