Yeah, what rustytruck said .If that channel is in good shape ..the body mount platforms are seperate pieces welded into that channel..
I'd support body remove body mounts and start by just removing the worst of the two platforms at this time if the channel is solid ,,then transfering the platforms or fab up new ones could be an option ,, If the top hole damaged as well then you could also clean up and weld in a repair washer...
If you have to replace the whole thing then removing the bumper and tailgate looks to be the best bet,,Do some cross bracing in tail gate opening..and pay attention to what your cutting ....
when installing plan your approch Plug welds from the top is so much better then doing overhead