I'd much rather start with a seized-up engine and bad running gear than a butchered cab, but I've always preferred doing mechanical work to sheet metal. Getting the cab back into shape is going to be a long, expensive process, but as others have mentioned, a first-series 55 truck with its one piece windshield is a pretty scarce item. Try to find a donor cab with the corner windows instead of a single rear window version, and have a professional body shop do the repair- - - -it's going to be a time consuming job, and your checkbook will need some serious CPR when it comers time to pay the piper!
Jerry
