Ha, my family's 1970's "Chevy" (Izuzu) LUV pickup with a 70 HP four cylinder can haul that much. It's all down to the spring rates.
to increase your load carrying ability you can add air springs.
I see these kinds of statements and only hope I never meet either of you on the road, towing loads beyond what the vehicle was rated for.
Sure, you can add air springs to the rear of a vehicle, but it doesn't increase your load capacity one pound....they only provide leveling for a sagging rear end. True, leveling the ass of a heavily loaded car or truck can increase stability but it doesn't increase the load capacity.
Why? Well, did you also increase the cooling capacity of your tow vehicle when you added the air springs? Thought not....and that's one important component of how a load capacity is calculated for a vehicle.
But that's not the most important part. Any vehicle can get almost any load moving....that's not the hard part. The hard part, and most important part in load capacities for consumer vehicles, are the brakes.
Easy to get an overloaded vehicle moving.....but if and when you have to get that load to panic stop because of some idiot pulling out in front of you and not moving.....then you're in trouble. And you're in trouble because your brakes weren't upgraded when you added those heavier springs or air springs with the mistaken notion you subsequently increased your tow/load capacity, and now you cannot stop worth a shit and plow into that car that just pulled out in front of you.....and when you get taken to court, you'll lose your butt because you were overloading your vehicle beyond what the manufacturer recommended and you're liable for that.
Sorry, but so much ignorance abounds with people who think they know about towing but really don't know much.....and those types of comments quoted above show just that; thinking you can simply increase the rear spring capacity and that magically increases the vehicle's GCWR (gross combined weight rating----the rating you get from adding the vehicle's GVWR and the towed trailer's weight.)