Well I not sure on Missouri law. **Check with DMV.** My best guess would be as long you keeping just changing frame and you own it. It might be ok. Most time you might not be able to put a older motor in the cab, the motor would have to be the same year as the cab or newer. Your going to have to registrar that cab if it not then meet what ever laws for motor and emission are for state and cab year. Make sure you have all your paper work when you go. The Other thing is are the two cabs have the same size humps. 73-80 frame and cab // 81-87 frame cab easier swap if you stay in the same years 73-80 body bushing // 81-87 body bushing 73-80 2x4 cab bushing are lower then 73-80 4x4 cab bushing // 81-87 2x4 cab bushing are lower then 81-87 4x4 cab bushing I hope this helps you out
77 1/2 ton Heavy Half chev 454,350TH,3.07rear driven 6 YR now changed back to 85 monte carlo SS 305 Motor as of 2004, june,30 inter changed to 84 GMC style, flatbed with a 98 van bumper.