This can be trimmed with scissors at the last minute. Remember, the official scale may not weigh the same as yours. Everyone has an opinion on where to put the weight. A car with more weight to the rear generally grabs more speed down the slope. But be careful not to put too much in the rear or you'll pop a wheelie.
Melted lead is dangerous and unnecessary.
No movable weights or mercury are allowed. Round weights found at the hobby shops and craft stores. This allows us to stick the weights out the back of the car. Paint them and tell everyone that they are jet engines or tail pipes. What they really do is allow us to get the weights as far back as possible.