Sure, here's your rebuttle:
The only way that YOU are going to be able to do 12's with 260hp in that convertible Mustang of yours is if you have someone FAST tow you down the track. Or you could try a FLYING 1/4 (not a dig), and you might get to the 12's then - assuming you start from your top speed. :taun:
Your car is a 2000 GT convertible, right? If so, curb weight is 3375 lbs stock, and 260 bhp.
http://www.miracerros.com/mustang/2000_specs.html
A rough calcuation on stock flywheel hp should be about a 14.5 on good tires, on a good track.
With the hp upped to 260 rwhp, you should see around a 13.7.
To make a 12.9, you would need a good launch and cut your weight from the car to keep it at 3375, and about 310 RWHP or 360 bhp.
I figured for a 17% driveline loss.