You need to find the break in the 02 sensor wiring first. You probably melted the cat, but if you don't fix the 02 sensor wiring, you'll burn up the next one immediately. It's not common on the modular motors, but if a clip fails, the sensor harness can fall against the exhaust. Of course there's always rodent damage possible. Or even just a cracked connector. Don't keep driving without getting this fixed, because it can cause additional damage.
I've actually had good luck with Landmark Ford when it comes to wiring issues. I had the same issue on my '94 once, and it only took them 4-hours to trace and and splice the broken wire. Actually any of the Portland dealers should have no problem with this, EXCEPT Damerow. Don't go there. They'll probably quote you a full engine replacement...
2000 E450 dually V10 wagon