I'd agree with Carl except for a few things.
1. The rear cab mounts are flexible, so it would be possible to get things off.
2. I hate working on my back, especially welding overhead.
3. The frame will be in the way of some places you need to get at.
My preference (and what I've done) is to remove the cab. It wasn't assembled on the frame at the factory.
1) The rear mounts are pretty much stationary if the front mounts are tight. I never notice my cab rolling around when I climb in my daily driver. Those rear mounts are designed merely so that the cab does not get crushed when the frame flexes, such as when driven off road.
2 &3) I hate working on my back also. Once I make all of the rust repairs from inside of the cab, the cab becomes a very rigid unit, giving me the confidence that nothing will get torqued out of shape once I remove it from the frame and stand it on the firewall so that I can easily get to the underside of the floor and the cab corners.
