It wasn't rated because it had no hitch. You could order a towing package from the factory. The towing package came with a frame mounted bolt in Reese hitch, trailer lights connector, an oil cooler, and an external heavy duty auxiliary transmission cooler, heavy duty shocks, and bigger brake drums in the back (11" instead of the stock 9" just like the station wagon because it used the wagon's rear springs as well) with a 3.55 rear gear in the differential. The car was then rated to tow a 5,000 pound trailer. You can add all of these things to build yourself a factory towing package.
The big B-body cars often used parts off of the lightest duty pick-up truck line (C-10's) to make the car heavy duty. Things like shocks and front spindles are interchangeable across the years and the truck (designed to carry the weight rather than pull it on a trailer) had bigger wheel bearings and brakes to accommodate it. Unfortunately the early trucks used a six lug wheel instead of five, but they can be redrilled to a different pattern if you wanted to adapt them to fit your car.
Remember it was corporate policy at GM to make as few parts as possible so parts off the truck line, B-body and Y-body (Corvette) often interchange (with the Chevelle, Camaro, and Nova also sharing many common parts).