The mustang is not a fast as the str-6 from the factory I know I've owned both cars the mustang is 3,900 lbs. but with a underdrive pulley and some other mods for very little money the mustang wind hands down in straight line 1/4 mile with its straight axle.
I love both cars but the srt-6 is not a drag car it more for as fun road course IF you want a fast 1/4 mile str-6 get ready to spent a lot of money!!
Drive what you have and be Happy from my model T to my 427 sc cobra.