There has been much discussion about how to swap cabs, especially since the VIN is shown on the cab. In Oklahoma you get Form 761A, which is Affidavit Of Assembly and Ownership for Automobile/Truck/Travel Trailer. When you fill it out you'll need original receipts or bills of sale for both the driver truck as well as the "new" cab. Take the form to the tag office and they'll send it in to the Tax Commission who will issue a new VIN and the paperwork that goes with it. You'll need to stamp that VIN on the vehicle, but will retain the original VIN plate as well as the Certification Label. Carry the new paperwork in the truck and you are set.

 

In oklahoma, they make it fairly easy, as long as you have the title for both vehicles, or at least a bill of sale for the donor cab. It also helps to have the donor VIN checked to make sure it doesn't show up on the stolen vehicle list before doing any work.... Otherwise, you could lose everything.
Getting the title with the donor cab, or buying from a licensed salvage yard, are the 2 "safest" methods for getting a donor cab. If purchased from a licensed salvage, even the VIN showing up on a stolen vehicle list isn't an issue, since it was likely just not removed after the recovery and subsquent insurance write-off.