Although I have a GMC now, I have owned F250's with both the 7.3L and the 6.4L. I was pretty impressed with both, but the 7.3L was pretty highly-modified. My 6.4L was a really good truck. I never had any trouble with the powerplant and it made smooth and ample power. I know the mileage was a concern of many on these engines, but it seemed rear axle ratio played a lot into that. My truck had the 3.73 rear axle and it would handle 11,000+ trailer loads here in the mountains with little trouble, so I suspect for most users it would do fine. I could certainly feel the load, but the truck would move it around safely and quickly. As for mileage, I never was able to do better than 18 unloaded with mine, and this would drop to 10-12 pulling. Interestingly, it would not go below that no matter what the weight of the load. I could get 12 pulling a 2500 pound small trailer/ZTR but would get 10.5 pulling 10K plus. I have never owned a 6.0L, but have too many neighbors and friends with them that have had trouble. The engine is just not well-designed, especially in the head gasket region with a flimsy 10 bolt design and short factory studs. Though many of these were replaced under warranty, my understanding is Ford did not volunteer to repair any that were not giving trouble, which if true, would not sit well with me. The 7.3L is a bulletproof engine, but requires heavy mods to make equal power to the new ones and finding a used one without a ton of miles would be difficult. I have driven a 6.7L and think it is the best of the line to date, but comes only in new trucks with big price tags. My opinion would be the 6.4L in a late model used truck is the way to go here, with the understanding that it is not going to be a fuel sipper, though none of them are right now and unless driving and pulling constantly the cost of operation is likely close to wash.
John M
