Yea, I don't think Cats will survive. I'm also a bit limited in height too as the floor pan drops a little into the space. I right now the main pipes next to the J-pipes are about 1/4" off the floor pan and heat it up quite a bit. I have 4" of frame height, so anything like a 5" resonator is going to be the lowest point under the car and be an issue.
Borla makes a 4" resonator (400939) that says to quiet it down 2-3db. I may still be able to fit a J-pipe in the space too. Need to do some more measuring today.