Using the 747 as an example-
1/ The cabin window frames are not installed on a Boeing built freighter
2/ Only 2 passenger doors are installed; the other 8 and their supporting structure are not fitted
3/ Purpose built freighters are often beefed up to allow higher max landing weights and zero fuel weights to make them more useful as freighters eg a 747-200F has a max landing weight 36,000 lb higher than a pax version of comparable vintage converted to freighter
4/ Areas recommended for cutting into the cabin are usually above the window belt to avoid trying to chop through the reinforced zone