That is a lot of weight to add to an engine. I would be worried about the additional stress that you are adding to the motor and gears. I know that it is expensive, but it you want to pull long trains, you might be better off in the long run getting another engine.
Many years ago I added weight to my LGB moguls. I put about 6-8 oz of lead in the cab. They could pull longer trains with out slipping, but it wasn't long before I started having to replace the idler gears that connect the motor to the drivers. It took many years before the light came on. Since I removed the weights and run shorter trains, I haven't had any gear problems.