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I don't have a crew cab, but I'm not understanding what the problem is here exactly.? Did you snap the bolt off or was the cab mount so rusty it fell apart? You should go get new hardware and order energy suspension body bushing kit. Once you get the mount off with a dead blow you will see the bolt hole in the bottom of the cab it just has a big nut welded on the inside that the long bolt threads into. I can't see needing to get access from the interior for any reason.
I understand what's going on now I re read this. I think you should cut the piece that's pointed flat and use an easy out with a map gas blow torch. If that doesn't work I would drill it out and re thread the hole. It wouldn't be hard to modify the mount plates to take a slightly bigger bolt.