I'm guessing that you'll have to change out the entire wiring harness forward of the firewall, but other than that, not sure. You might want to post this in the Trucks - 18 wheelers forum.
Could be really ugly. If the Detroit (as configured) was not offered in the Pete. If it came out of an 04 Pete and you have all the do dahs, it's a bolt in. The path of least resistance may be to fix the Cat.