It will likely be considered a reconstruction. It will look like a 40 Dodge PU. All possible receipts help. Expect to get a different story from each DMV person you interact with. Stay polite and appreciative of their help. They do not expect the be treated nicely by the public. Read the rod-building rules for your state, and know them better than the person on the other side of the counter. Have a copy with you, and highlight any arguments you expect to face. The rules should be posted on the DMV website. If you have a vin on the cab, have the DMV, or a cop if you know one, run it for you. It will likely not show up at all. If it does, "repair the rust" in the firewall, so it no longer exists. (Oops, I didn't just suggest that you do anything illegal.)
I am facing the same issue. Lesson learned, I hope. Best of luck.