I would bend the poorly-made headers before I misaligned the engine. Steel tubing is a "plastic" material and the headers cold be bent 1/4" or more quite easily to solve your problem. Protect the header tube from the gearbox with thin plywood, pull hard into the geabox, then back the other way. You'll easily get 1/4" clearance this way, at "$0" per inch.
Well the mounts are installed and what a pleasure. In total I would say that the installation time was about four hours in and out. Once installed It took,only a slight push and I got the clearence that I was looking for. And without damaging the ceramic coating on the headers. I did not use the lowering portion so if anyone can use them they are yours for the asking. All you pay is the shipping.
I have built many high performance cars in the past but this is my first Mustang. I have gotten more aggressive on two occasions in the past. One resulted in crushed tubing the other a crack at the flange. Neither set were poorly made as another member stated. One was Hooker the other Doug Thorley.