Ok just a quick update I took the day off Wednesday and visited a friend who was a Cummins master tech. The first thing he went to was the cat. We checked differential pressure and it was definitely plugged. So we knocked the crap out and shoved a 3.5" pipe back through.
I have racked up another 300 miles since then and it's getting about 16mpg now in the city!
Thank you everyone for your help