My personal experience - MAC doesn't fit well with a Tremec 3550 - the exhaust pipe hangers are in the wrong places & the cats rub against the rear crossmember.
Do a search on MAC - one Stangnetter couldn't pass emissions with his MAC cat H pipe & had to swap it for another make.
Mine fit fine on my '90 GT. And I pass inspection. In Upstate NY you have to have the required parts of the emissions system and they have to apear to be working. If so, you are good. That changing for '96 and up cars soon though.
The death rattle is when the element inside the cat breaks up into pieces and rattles around in there like a rock in a coffee can. Very embarassing sound.