I have had a 5500 4X4 CC since I bought it new in 05. Not exactly what you are looking at. My gear ratio is only optioned at 5:19's. Mine is loaded similar to an SLE pickup, power everything, cloth seats.
The upside: Incredibly stable towing platform. Outstanding room inside the cab. We pull and carry at or over GVW and the truck is seriously overbuilt. Very good turning radius, yours isn't 4X4, but its extremely capable offroad for a big truck. Huge amount of ground clearance. They are detuned. Mine specs like an LB7. No issue however it pulls hard, the low gearing helps.
Downside: Very rough riding, mine is leaf spring only. The inside has a lot of plastic, I am not sure what they may have done with the conversion units like your interested in for comfort. The factory seats are ok, not great. I would make sure it has an exhaust brake, mine did not from the factory, they were having issues with the factory units at the time. I may put one in yet. Mine has been decent on reliability. The head gaskets went out, but I attribute that to the operator. Some electrical issues, nothing other than the head gaskets that was a big deal.
Overall, they do well with what they were designed to do. The engine and tranny are solid, and the frame and suspension are true medium duty class. The biggest let down in the my truck (not yours) is the comfort level and some poor interior quality. However I think the conversions have replaced most of the interior.
