I have always slammed my Mustangs and this one is no different. 3" in the front and 4" in the back. As a result, I have had a few different brands of headers and have seen many others over the years building Mustangs and doing custom exhaust.
In my experience, Hedman makes the best header for tucking up underneath close to the frame. Performance wise, all headers are the same, although manufacturing varies and quality of the product varies too. But as far as ground clearance goes, Hedman has it by far. Made in Mexico now, so the material is ok, as is the workmanship, but not as nice as say Sanderson.
However, they tuck way underneath, so much so that you will need to add some extra supports to your speedo cable so it doesn't get burned.
Good luck on your quest for clearance.