You aren't going to hurt a cast crank at 6000 rpm, especially with a lightweight piston. You can internally balance a cast crank engine if the bobweight is low enough. Probably needs to be in the 2500 range but I don't know for sure anymore. Figure out what your bobweight will be and go pay the machine shop for a balance job and then you'll know for sure. If your crank is heavy at the bobweight of your proposed parts then you'll be okay. If it is light then you'll need to find lighter parts or budget for heavy metal.