you may want to add the vin# in case someone bought it with a salvage title. Also car fax your vin and see if it comes up after you last drove it. Just some ideas to pursue
How long exactly has it been missing? Were you ever able to figure that piece out?
Who owns the place that it was stored?
He should be able to tell you exactly when it arrived and when it was discovered as missing. If not then THAT person is probably responsible for it's changing of location or at least knows who it was that came and got it (probably permitted this to happen).
Have you filed a police report? Cops can't ask anyone questions unless you have.
My feeling is that this thing has already gone to Mexico and has been stripped for parts.
Did you have insurance of any kind? Storage insurance?
Call the local junk yards in the area that you think the car may have last been and have them search the VIN to see if they scraped your car. Unfortunately most 5.0 Mustangs are worth more when parted out rather than a whole especially if the car has no title. In my area there a lots of vehicles sold with "bill of sale" only and no title. Those vehicles become near worthless as a whole so they get torn apart and the body sold to a scrap yard or dumped in a field.