I'll make a case for the T-5.
First off, don't confuse the early T-5's (rated for 260 f/p tq) with the late T-5Z (rated for 330 f/p tq).
Second, close ratio T-5Z's were not installed by Ford in any Mustangs so upgrading or new purchase is necessary.
So far, 15,000 miles on T-5Z filled with Moble 1
synthetic ATF behind stout 351W, and no problems. I love the way this trans shifts.
A lightweight Cobra will spin the tires before putting enough torque through the drivetrain to damage the trans. The one problem with the T-5 is the shifter forks. The shifter MUST be a positive stop design which the stock one isn't, and aftermarket ones (Hurst, etc.) are.