Giving the year 70 gives some insight to this subject. What year was the truck you sold? You said F-100 and the only years interchangeable with 1970 is 1967-1972. Well the floor pans will fit all the way through the next generation as well but thats about it. I have owned several 67-72 F-100's. The frame does have the vin because the other VIN was on the door. If a door got swapped then the VIN was changed and now like most people have a title to a truck with the wrong VIN. Tell him to talk walk anyone from
www.fordification.com. Keith on the site is a Bumpside (term to distinguish the body style) GURU. I have seen this problem several time because of this though. Does the title match the door or the frame? If it matches either one he should not have any issues. Check out the site and ask around this is a known issue and this is not the only series of vehicls with this problem. If I recall correctly you almost have to remove the body to see the VIN on the frame though. Let me know if I can be any help.
My userID is Gmcjimmy88 on there as well. Whatever you do dont ask about putting a SBC in a Bump for your first post LOL. Still getting shit over that one LOL
Some more info borrower from fordification.com
"In addition to the V.I.N. on the driver's door data plate, the number is also stamped on the frame in two different places, both on the topside of the right frame rail. The first one is in the engine compartment, just below the alternator on two-wheel-drive pickups or just behind the front axle on four-wheel-drive pickups.
The second number is stamped just under where the seat would be located. You'll likely need a flashlight and small mirror to read this one.
These frame stamps are usually hard to read, so be sure to grab a wire brush and some rags to clean the grime off.
Often during the course of normal life, the driver's door might have been replaced due to an accident or rust repair, but the V.I.N. tag on the donor door is sometimes not replaced with the correct tag from the vehicle being fixed. It might be some time before anyone notices, but as luck would have it, it's usually a policeman or the Department of Motor Vehicles (DMV) who catches the error. Because the V.I.N. on the 'new' data tag doesn't match that listed on the vehicle's title, it creates problems for the owner trying to prove that he actually DOES own his truck and that it's not an attempt at deception. Therefore, you need an alternate source for the vehicle's V.I.N."
Link to the above info.
http://www.fordification.com/tech/vin_find.htm
Hopefully this helps you both.