We have an American Bulldog and I looked into this to get some of the "need to pull" energy out of him. I decided against it after I found out it can wear out the dog's body over time and shorten their lives. Most of the pulling breeds love the activity, but it has to be done right.
You do have to invest in a few really good pulling harnesses. Just search "dog pull harness" on Google. Buy the best you can afford. Dogs should
start wearing a training harness from puppyhood on. It's better than a choke collar anyway, which only strangles the dog. A good harness works as an "emergency break" since it's always easiest to stop a dog by controlling the power in their chest muscles.
Here's an article that has some good advice:
Weight Pulling Competition for Dogs
I'm not sure what the rules are for mutts, but this is the code for American Bulldogs:
Competition Rules
Here's some free information:
Weight Pulling Guide
It takes a lot of patience to properly train a dog to be show quality. It's also important to do the training properly so the dog does not get hurt. I've heard horror stories of dogs having to be put down because they were damaged beyond repair by an over eager owner who gave them too much weight too fast or were too cheap to buy the proper equipment.
A professional show will disqualify a dog for not having the right harness.