There is no dash mounted ID on a 65 -67 Mustang. Look for the VIN on the driver's side shock tower. There will be a cut out area along the top fender sheetmetal with the car's VIN number. There is also a tag "Warranty Plate" on the driver's door that contains the VIN, and the coded numbers/letters for the car's date (which isn't actually the date of build, but the date that the order for the vehicle to be built was recorded) The actual build date could be a day or two later or maybe even more. The other info it has on the plate is the codes for Body, Color, Trim, DSO (District Sales Office), Axle and Transmission.
You will need to compare the VIN on the shock tower with the VIN on the door plate. If the door was ever replaced and the original tag for your car wasn't transferred, you'll never really know the date for sure. The records for the 64 1/2 to 66 Mustangs were destroyed in one of Ford's moments of sheer stupidity. The records for 67 and up were saved, fortunately. The Warranty Plates on the door being for the wrong car is quite common if the door was ever replaced. Just hope that both of the VINs match, the one on the shock tower with the door's Warranty Plate.
The only chance to narrow down the date and the other info if you don't have the plate is if you were lucky enough to find the piece of paper called the "Build Sheet". Most times they're gone, but if you look under the dash, under the carpet, attached under a seat cushion, front or rear seat or any other nook or cranny in the interior, you may find it.
Another way to get an idea of the date is to find a Mustang with a VIN close to yours. | 
65 Mustangs didn't have dash tags.
If you could tell me the first two and last six digits of your vin, I could probably help you out with a build date.
I knew Veronica could help you out, she knows everything about our Ponies you could possibly imagine. 
