Replacing moderate-sized sections of the floor pan can probably be done with the cab on the frame, but swapping the entire thing needs to be done with the cab removed and on a jig or a rotisserie. A cab mount should probably be swapped with the cab removed and reinforced in some way to assure the mount is installed in such a way to prevent distortion of the cab when the new one is bolted to the frame mount bracket.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!