I have often thought about owning one of these but they must be quite rare now. Look after it well!
There is an interesting problem to solve. If you are plumbing in all 6 brakes, you will need a larger diameter master cylinder, which will in turn require more pedal force. I don't know what braking system you have at the moment, but it may be time to think about a servo. Not sure about which brake limiting valve would be correct for the rears, maybe using the smallest slave cylinders, 0.5 inch, could be a good place to start. It might be advisable to find out what other Hustler owners, or for that matter, 6 wheel Scamp owners, have done. There is at least a Scamp owners club. This also affects 6 wheel Limos and similar conversions, and is somewhat challenging. If you get the brakes working properly on all 6 wheels, i.e. with plenty of rear braking but no wheel locking, please do let us know what you did, as it may be helpful to others.