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You're right it was a significant amount of bondo on the cab corners. If you were to ask my opinion I would say cut it out and replace it. New metal is better than old metal unless the old metal is in good shape. I started to sand mine and after about 4 discs the cab corner debate was over, they had to go.
Mid fifties had the corners for a good price. I'm still learning how to weld and my impatience got the best of me otherwise I would have done them myself.
I had no bondo in the corners themselves but they were rusty so I cut them out and am in the process of replacing them. The bondo I found on my truck is on the rest of the cab back and appears to go all along the back between each vertical indention. The cab itself shows no signs of damage from the inside so I am puzzled as to the reason for the bondo and thought it strange your cab showed a similar pattern or appeared to in the pictures.
Thank you for the positive encouragement @ Peter Bird and Jdanthony. FYI my rim size info is found early in the thred if you have a little free time to browse through. They are 20 inch intro twisted vista with custom offsets.