Or just drive it.
I made this file to look at speed vs gear ratio while considering options for my old car.
https://drive.google.com/open?id=0BzsCcQ7N5imvbXF1d3NlRm53RWc
If you bought it new and haven't touched it, it has 3.31's
No telling what kind of life it had if you bought it used, though. I know my old car has the stock housing, and probably a 3.31 sticker, with 3.73 gears.
I paid well under 30 for my base gt 2013 in 2012... I'm sure the base GT will be near enough 30k on msrp that once the hype dies down, they will be able to wheel and deal into the 20's.
And look at the price of other cars in the era. Dodge caravans were pretty close in price in 1991, right...
Call around. I would be surprised if you couldn't find a seat delete somewhere.
I'd start with Northcoast, guy is local to me and has a warehouse with many things in it.
http://www.nthcoast.com/2005-2014-Mustang-Rear-Seat-Delete-Kit_p_1256.html
I know two people with sponsorships.
One because he has a pretty sizable following on instagram and gets a lot of attention for two specific companies that he was buying from at their normal prices until about his 4th or 5th car. And his shit isn't free, they just give him SMOKING deals on...