I am new to this type of body work and with my first repair I learned very quickly you must take steps to protect/preserve fitment when you start replacing sheet metal. My first sheet metal repair was replacing the cowls. I got lucky in the fact I was able to make some adjustments after the fact and get a decent fit at the corner of the hood and fender. Since then I have watched a lot of YouTube. The detail the guy doesn't go into is what is involved in doing the repairs off frame. I feel doing the actual repairs off frame would be a lot easier in regards to access. The big "but" here is preparation. Just like everything else. Because a significant part of the structure is gone or compromised you would need to build supporting framework before pulling the cab from the frame. If you do that and have a rotisserie and the equipment to do it and the time, pull it off. for me, the back yard mechanic it is easier to do it on the frame.
I did just watch a cable vintage car restoration show last night and they were repairing some floor pan replacement work done by the owner of a 1951 3100. It was a nice looking truck but when the owner cut out the front floor pans and rockers he did not support the door opening. He did not realize the cab slightly collapsed at the bottom closing the distance between the door opening at the bottom of the door. After he welded his new pans and rockers back in his doors would not close. He brought it to this shop to repair his repairs.
