In my limited experience, it depends a lot on the state. In PA, the registration VIN seems to be the driving number. If the body VIN doesn't match, you may or may not catch grief from your inspection shop, but PennDOT may or may not really care - as long as the VIN matches the engine for emissions testing (if required). Now selling it to a resident of another state can cause issues. When I sold my Frankenstein '95 Cummins ('95 Cummins 4x4 drivetrain and dash/VIN installed in a '96 V10 2WD chassis & cab) I was sure to mention the VIN discrepancy up front to potential buyers. The guy who bought the truck was in MD where they have different registration procedures and the DMV there gave him a hard time about the VINs. To the point where he ended up keeping the registration in PA at a different residence.
