I think from previous posts and discussions we have clearly established that a TRUE "A code" factory GT car DID use the same chassis metal build as ALL HiPos (obviously built from Spring '65 on).
What this means is that originally the HiPo was the only Mustang avaiable with a dual exhaust set up from the "factory" - this included the relocation of the rear brake hose and various other metal changes like the rear muffler supports under the seat and depending upon when the car was built different frame rail supports for the tail pipes.
Now add to this that all HiPos received the 9" rear end - therefore they received a different axle bumper plate as part of the floor pan.
Now keep in mind that FORD would have been thinking of manufacturing efficiency and COST. So when they launched the "GT" cars with dual exhaust included, they needed a metal frame set up for this. They didn't "create" a new one model only frame build for the "A code" cars - they aready had what they needed with the K code metal set up.
This is another way to confirm a "FACTORY" GT car if it is an A code.
Somebody can correct me if I'm wrong, but based upon my 25+ years working with early 'stangs only the K code had "factory" dual exhaust and a 9" rear end - only a factory A code GT had "factory" dual exhaust.
I have never seen a documented plain Jane A code or C code car with factory duals or a 9".
Back to the point of the original post ---
--- franKcode --- You my man have a rare conversation piece. If everything checks out as you have described you have a very rare factory mis-stamp. Guard your documentation with your LIFE! And enjoy the car!
If you do a full resto on the car I would document every aspect of the metal frame in raw metal as part of the proof that the frame is virgin and only the VIN stamp is incorrect.
One word of advice though - CAREFULLY approach this subject with your local DMV. I know in Georgia they are a little picky about the VIN stamp. That is what they use to ID the car - door tags and other documentation don't count. They even cut out VINs from theft recovery cars! and assign then a new "Georgia sequential VIN". Just make sure that if you ever sell the car that the new owner is aware to be careful - laws vary state to state.