Oh mama. While I can't answer your exact question I can tell you of my recent experience on a similar issue. I recently did a frame off restoration on a 57 Belair for my neighbor who developed some serious health issues. While I was providing the labor for free (I just like doing these sorts of projects and he asked me to to it for him) he owed me a considerable amount of money for parts when the car was completed. Since his health issues were leading to financial challenges he decided to just give the car to me. Great except he subsequently could not find the title so I had to get a bonded title. Sounds simple...
Part of the process involves getting a form 62-A through an inspection by the local "auto theft task force". I am in Denton county so my options were to go to Dallas or Fort Worth. I chose Dallas since they do inspections twice a month versus once a month for Fort Worth.
Upon my arrival the first sheriff to inspect my car immediatley announced that the VIN tag had been removed and reinstalled. After some considerable discussion with him and other sheriffs it was finally agreed it had not. Next they put the car on the lift and announced that the car was on an aftermarket frame because it was not rusty and because another expert working for the Texas Insurance Board had never seen a 57 chassis like this one before. After arguing with him for some time that there were two styles of chassis for tri-five cars, California and others, he finally went on the internet and confirmed this is true but still maintained the frame had to be aftermarket. Yet another sheriff piped in that his dad had a 54 Chevy and the frame was rusty so it was impossible for my frame to be rust free. At this time they started pointing out the sway bars, disc brakes, and polished/restored stainless etc as further evidence that I had built the car from scratch. These guys had absolutely no idea what they were doing when it came to classic cars. I am telling you all of this to prewarn you what sort of experience you could have if you go there with a car without a VIN tag. They could very well declare the car stolen. They are incompetent enough to do it...
I am not trying to scare you here, I am just trying to make sure you do all your homework well in advance. Depending on where you live you might also want to try Fort Worth since it is hard to imagine they could be any less competent.
Regarding your question about the reassigned VIN, I am reasonably certain it will not be a VIN for a 67 Mustang since they have no way to verify that they are not duplicating an existing 67 Mustang VIN. You should also check to make sure the title you get will not be bonded. As far as I know you cannot export a car with a bonded title. You should check this.
Good luck I hope you have better luck than I did.
The form and rules are here:
http://ftp.txdmv.gov/pub/txdot-info/.../2000/5-00.pdf
Don