This is a GT rear exhaust hanger (different than a V6 hanger)
Get a set of GT rear hangers (Ford Part XR3Z-5A246-AA).
Install them on your Mustang using 5/16" x 1" bolts, washers, and U-lock nuts.
Just slip the U-lock nuts in the two square openings and bolt on the hanger.
You need some kind of alignment jig to determine where the exhaust openings should be cut.
Measure your GT-takeoff exhausts, and make the simple jig illustrated here.
A piece of 3" diameter sewer pipe serves as a stand-in your exhaust pipe.
The 3/8" aluminum rod duplicates the location of your rear pipe hanging rod. You could use a 3/8" hardwood dowel.
You can check your jig by sliding it onto the tip of your new duals. Does the rod line up?
You won't be able to put the 3" tube on the jig until after you've cut the lower lip of the bumper.
Everything looks o.k. Time to cut.
Here's the results after using the 1/4 inch "saw drill". The cut was made with the 3" tube removed. It was then installed to check alignment. Looks pretty close. The edge is rough, but the saw drill cut quickly, and was very easy to control.
Here's the opening enlarged using sandpaper and a 4" diameter sewer connector as a sanding block. The sanding block (tube with sandpaper wrapped around it) is pushed/pulled back and forth along the same path the exhaust pipe will travel. It produces a very smooth edge, and a perfect radius on the curve.
It doesn't even need painting - the edge cut is that clean.
Before you do the drivers side, you need to to loosen the hanger, and pry the V6 tailpipe out of the way. Two pieces of 2"x4" lumber wedged between the tailpipe and the unibody does it.
GT-takeoff dual exhaust system installed. Click here to see how that was done.
If the cutouts don't line up perfectly, or if you want them to be larger, it's easy to fix that now.
Just slip the 4" pipe sanding block over the exhaust tip, and remove some more material from the bumper.