All the cabs are dimensionally the same (a COE has many unique items) Some things you will want to keep are the parking brace from the driver's side (configured differently on large trucks with stick actuated brake). There will be some PTO items and brake parts you will need to swap. Also some firewall related valving and parts if your truck has a 2-speed rear. I would recommend if you purchase the cab if you keep them until you have transferred everything. Comparing them you will see all the differences. The other cab could have other differences but without a year I cannot be specific. As stated 51' is a one year only door design.