Officially, probably around 6800. Can it do more, sure. You didn't say auto or manual, although my opinion is manuals are more durable for heavy towing. My 88 250 5.8 3.55 will haul heavy, just not in a hurry. Going to 4.10 will help hauling ability, empty is where you may notice a bit of mileage loss. Driving style and speed are also factors. Will it make a massive drop? I doubt it. Will it tow better? Absolutely.
With my personal experience I would agree.