Going to throw in my nickels worth here, co worker just bought a 21 2500 HD cclb 5th gen front end.
I have 16 2500 HD cclb, so we parked them side by side and gave them a good going over. And what we can tell is the chassis is the same, the cab, doors, bed, suspension, the seats.
What we can tell what has been changed is the dash (should be able to put into 4th gen) we think the interior should be able to be swapped with little to no modification.
Obviously rear bumper is different, but they may have changed the rear bumper mounts and front mounts as well. The front sheet metal should bolt up with out to much hassle as far as we could see, the body lines are identical on the fenders, the hood has changed (I like his better) but should bolt up as long as you change the whole front clip.
So between 4th & 5th gen 2500 HD's these trucks are alot closer to each other than we realize.
Oh, forgot the most important thing, I have a 5.7 and his is a 6.4 with a 50 GALLON gasoline fuel tank, yes you read it right, no I did not typo. The tank straps are identical and mounts in the same spot. I am now on the hunt for this tank.