If you're using a 451, I'd say 1,600 pounds is your maximum load. A few years ago I did a torture test of sorts to see what kind of towing a smart could do. I got it up to 1,700 pounds on some trips carrying motorcycles or moving out entire apartments. Did this testing for about 15,000 miles and the car did great.
1,600 pounds can be rough on the car if you push it too hard. Avoid hill starts and keep speeds no greater than 60 mph. Lastly, drive it like a truck. Keep long following distances and do long brake runs.
NOW! I'm not the police, a lawyer, or an insurance company. These cars were never approved for towing, so if you get into a crash you could face a legal mess. Keep that in mind.