Dixie Gun Works used to custom make bronze round ball moulds in any requested diameter. Not sure if they still do or what their max size is but I believe it was in the .75 caliber range.

6 bore - wow. That's a lot of metal - you'd empty a Lyman furnace in short order pouring that slug. These folks might be able to help you:

http://www.2-bore.com/2boretech.html