If the bed is off, you should be able to do that. This is not from experience, just the fact I have pulled my cab and bed off the frame and have ordered new cab corners to replace them. So I have simply seen what things look like when they are apart.
I pulled the cab more so to get at the frame than the cab corners, but in the end I think I will be glad I did as it will be easier to do the toe boards, brake master, and trans mount, etc.
http://www.brotherstrucks.com/techarticles.asp Brothers has a tech article on doing the corners, but they have the cab off the frame. But if you look at the pics you can see the frame mounts considerably inboard of the corners and with the bed off I think it is doable.
