My own experience with my '99 4.2 with 4-spd auto and 3.55 rear end: I towed our small "hybrid" travel trailer - gross weight about 3500 lbs. - from North Carolina to Key West and back. Obviously no mountains or even hills to speak of, and Interstate almost all the way, except for occasional side trips and, of course, Highway 1 through the Keys.
Yes, the truck did it. Yes, it was working hard. It would hold about 65mph on a straight and level with no headwind, with the "overdrive" turned off, but any wind or hill would slow it down. Mileage was 7 - 8 mpg.
Now, lest you think my '99 is an "old rag", it's in great shape. Unless you look very closely at the paint, you might think it's perfect. The engine was completely and professionally rebuilt about 1,000 miles ago, and runs perfectly. It's actually quicker off the line than a friend's 2008 5.4! The transmission is fine, as are all the other mechanicals and electrical systems.
After doing some research, I find that Ford rated this truck for a "Standard Towing" weight of 2,300 lbs, with a Maximum of 7,300 lbs. In theory, at least, this means that the truck should be able to tow 2,300 lbs. with little or no problem, and while it will tow the 7,300 lb. weight, it would be best to limit it to short distance towing. Weights in between these values will obviously affect the truck's performance accordingly. The towing weight of my trailer was more than 1,000 lbs. over the "Standard" weight. This is 20% of the "towing range", and it gave a rather severe workout to the truck.
I don't think I want to do that again. I'm going to limit any further trips with this truck-trailer combination to relatively short distances; those that can be covered in a days drive or less. Going back to the original question in this thread, I don't think I'd want to try to tow #6,000 for very far with a 4.2!
I love my old truck. I fully expect it to last at least another ten years, with reasonable care and barring any disasters. But, I will respect its limitations in the future, and not subject it to work beyond its capabilities.