Cab Corner Replacement Questions

[farmallboy15]

So I have a 1970 GMC. I've gotten to the point where it is just rust repair left. I think I have the rockers covered but with the cab corners I have some questions. I attached some pictures of what I'm dealing with.

My primary question is what does the cab corner weld to inside the cab? The "backing plate" is too far gone to see.

Does the cab corner weld directly to the outer cab floor extension? I have the floor extension on the passenger side but still need to get one for the drivers side...