I believe the "standard" Bally 6000 is the design you have. Although, Bally 5500 glass kits will fit the cabinet, they will cover up the speakers, since they do not have the speaker cutouts. People have done it, but as to be expected, there is some slight sound muffling/distortion.
Have never seen/heard of anyone customizing a 6000 by adding IGT speakers externally, but crazier things have happened. I'm guessing its possible.
Bally did not make a "conversion" kit to move the speakers. It is not common for a manufacturer to make a retrofit part for their current machine platform to be converted back to the previous generation of machine type.
Hope this helps some.