Are you sure that the friend of your friend actually had a 460? Gear ratio can have either a positive or negative impact on how a vehicle handles a trailer, so if he had more highway gear than towing gears he could have had some issues. Either way I can't imagine that a fuel injected 7.5 liter engine would have any trouble pulling a small trailer like that. Without having all of the information and assuming he really did tow a 6k trailer and had trouble with it I would think that he probably had the 351 instead of the 460. My Uncle had a '92 f350 with the 460 in it and it pulled like crazy, very stout engine. Anyway I wouldn't be the least bit concerned about this towing combo. Just get out there and have some fun.
Eric