After studying this problem I came to conclusion that only a rear lower swap was gonna really repair this properly. so I made plans to cut the part from a black 84 cab up in the woods I have that got wrecked. But upon closer inspection the black cab was wrecked on the right rear not the right front as I remembered!
So this blue cab is going to the parts pile for now & I will use an 85 GMC cab I have. I hate to waste this cab but maybe I can fnd a donor later on.
I went to pick a part today to look at the 73 I cut the cab top from but it was gone...