Up until less than a year ago, I was a licensed salvage dealer. It is common to put a replacement cab on a damaged vehicle. I am also in the trucking business and several of my trucks have had replacement cabs installed. I would just do it. If you ask some enforcement agency, they may want to show off their authority and tell you all sorts of BS. As a salvage dealer I was audited periodically. I just made sure I could document the origin of the cabs I installed.