I had a 96 3500 which never idled more than 5 mins in its whole life..so i never had cat problems in 400000 miles..it always had good power..seen other turcks with plugged cats and it just slows a diesel down to a crawl eventually...ford used to have ''spark arrestors''at the end of the exhaust...little holes like mesh..they would carbon shut and with a hammer and a punch you could give it a tuneup and it would run like brandnew..if a diesel cant breath it cant run..thats why turbos are its best friend...i'd have a muffler shop make a pipe to size for replacement and see if you need to do anything else...also, dont forget the fuel and air filters

 

How a bout a 1.5 year later "follow-up"...did those things help bring your power back up?