I agree with Bartamos, Carl and Mark.
As long as the frame isn't' twisted or severely bent, it's the reference for the cab floor. I'm doing my repairs off the frame. The door frames were still intact when I took them off (rockers, etc. still connected, so I put bracing on them before removing the rockers to maintain the door fit. I put the cab on a rotisserie so I didn't have to do the work laying on my back.
Here is how I did that. The rotisserie provides some door frame bracing as well.
Making sure the doors fit correctly is the first step, then bracing so that alignment isn't affected, whether on the frame or off.
Good luck with your repairs.
