Repairing my Cab

Hello, first post so sorry if this has been asked before.

I have a 2006 Western Star 4900FA, love her to death. Mechanically I've been over everything, rebuilt the power train, rebuilt the engine 160k kilometers ago, eton referbished transmission. Referbished differentials. She has never let me down.

Problem is the cab is showing surface rust around the windshield and doors. One door has already been replaced, I replaced the other soon.

I'm not sure about the terminology but the crossmebers under the floor of the cab look really rotten. Not the frame but the cab assembly itself. My question is can this be fixed without lifting the cab off? Can it be fixed at all or do I need a new cab assembly. Has anyone done a cab assembly before, and how much did it cost, I know I can get some decent used for under 5k.

Any information helps.