Point taken.
However...
GM made a 6-lug 2500 series truck from 1988 to 1999, with a 7200 lb. GVWR and a towing capacity of 8500 lbs. The Cadillac J55 front/rear suspension and brakes (which are now in this wagon) also have a GVWR of 7200 lbs--because they're essentially the same components as the 6-lug 2500 series trucks, except for 5 lug wheels.
On top of that, this wagon also has a heavier duty transmission and heavier duty tires than both the 6-lug 2500 series trucks and the J55 Cadillac. So...I've been quite satisfied with what it can do.
I seriously considered changing it to 8-lug front rotors, rear axle shafts, and wheels, but ultimately decided that it wasn't worth it. 8-lug wheels are only necessary with significant
payload, but aren't really needed for heavy
towing. GM has rated 5-lug rear wheels for at least 4,000 lbs. GAWR. The rear axle on this car sees about 2300 lbs. empty. That means I could have 1000 lbs. of tongue weight (10,000 lb. trailer) AND 700 lbs. of passengers and cargo over the rear axle before reaching that limit. Or, I could have 1,700 lbs. of payload over the rear axle...but I'm not sure why (or how) I would ever carry that much weight inside the car. If it were a pickup with a bed, though, the situation would be a bit different.
Let's also remember that the rear axle itself has a 6,000 lb. GAWR, and the tires have a combined 5,358 lb. GAWR. So the five lug wheels are by far the weakest link in the chain...but they're not that weak to begin with, and again, only really affect payload, not towing.
I made the frame of the Kammback out of thin strips of aluminum, and it's screwed/bolted to the rear glass hinges at the top and to the window strut mounts at the bottom. The top and sides of the Kammback are coroplast, attached to the aluminum frame with screws. The gap between the top of the Kammback and the roof of the car is smoothed/sealed over with a piece of Eternabond tape (that flexes with the hinges when the glass is open). The sides of the Kammback attach to the car with rare-earth magnets--again, which detach when the glass is opened, and automatically re-attach themselves when the glass is closed. It took me a few tries to settle on this design, but it's durable, works great, and was fairly cheap/easy to construct.
-Funkhoss