I drove a '99 7.3 w 6spd manual 150,000 miles and a '05 6.0 6spd 75,000 miles. Not one problem with either pulling 13,600 - 17,000 (camper + boat) on short and long hauls. 7.3 has lots of low end, 6.0 weaker on the bottom end but comes alive with a few extra RPM's. I think with an automatic the 6.0 would be a better truck than any 7.3 pulling 13,000lb, if you don't mind bottom end lag, the 6spd manual is bullet proof if you can drive it properly. I would shy from '03's, not looking for a fight '03 owners just my opinion, '04 and up will get the job done for ya.