Unfortunately, you're a year too early to be able to change this in a tune (I'm in the same boat).
Gears might be possible, but there's some caveats:
4.10s on stock tires with the stock main speedo gear (8 tooth) requires a 23 tooth sender gear; what I've read suggest that this combo will destroy itself.
If you're running a manual, you should be able to change the main gear (to a 7 or 6 tooth gear), which would allow the use of a smaller sender gear (21 and 19 respectively). To do so involves pulling the tail off the transmission. From what LMR says, the auto trans cannot swap this gear (I'm no 4R70W expert though).
The other option is a speed cal unit, which is installed between the speed sensor and speedo, and allows adjustments of the frequency being sent. I've not see any Mustang specific ones for sale lately, but someone brought it to my attention that there are generic ones, and I have found a couple that output the correct format for our cars (8000 hz per mile). So far, this is what I'm considering:
Dakota Digita Speedometer Signal Interface From what I'm reading on it, this should work, but I do have to say I have not done this yet (I've been waiting to do gears until I could solve this problem).