Sounds to me like you got your money's worth out of those gear boxes. I use Tamiya Panther gearboxes in my HE tanks. This takes some custom work, they are not drop in just as any other gear box wont be. I would just replace the boxes you have with the same type. This is the only way you will get a plug and play swap. Again, it sounds like your  original boxes held up well. Why change now? Hard Driving will shorten the life of any gearbox, bearings on the final drives or not.