Throttle control is the name of the game. My 8 year old son drives our Monster GT conversion and has adapted well to it. He can make it wheelie on demand, but also doesn't have any trouble keeping it on all fours either.

It isn't necessary to add any weight. In fact, I've found that it doesn't help. It just makes the truck handle like a tank.

Go with a moderate setup, like a mamba max with a 10XL. It's still going to wheelie, but won't be as outrageous and some other setups.

Just because your car can do 140, doesn't mean that you'll do that all of the time. It is nice to know that you can though.