A buddy of mine and myself managed to get 3" over the axle of a 70 Mach 1 so I think you can do it also. If I remember correctly we ordered premade parts and I think it all might have come from Pro-Motorsports as we used one of their x-pipes at the same time. The only issue you might have it interference with the shocks but that can be worked out thruough carefull installation and maybe a little hammer work on the pipes if it just won't go. GOod luck.
Dr. Gas at less than $200 but perhaps only for 2.5 inch
I got on Mustangs Plus' mailing list and received some catalogs in the mail. One featured a 69 Mach I painted in Signal Green and the owner's pipes ran over the axel. Looking through the M+ master catalog, it occured to me that one of the Dr. Gas exhaust kits they sell also does the same. I am looking at page 234 of their 02scat at Dr. Gas part number 02979 or 03009 for 2.5 inch. Maybe they also have a 3 inch. Rots o ruck.
Flowmaster makes a kit for 67-70s that is pre-bent mandrel with no pinches. You can get them in any dimension from 2 to 3 inches. I bought the full 2.5 kit, but my mufflers are Magnaflows.