Help Removing Secondary Shaft

I'm on my journey replacing the starter clutch on my 1983 KZ750 Spectre, and I've gotten to the point where I need to screw off the secondary shaft nut so that I can remove the secondary shaft. My problem is that I can't get the shaft nut off. I've tried having my bike in first and holding the brake while using a breaker bar to try to loosen the nut, but this just results in the whole shaft turning and what sounds like a seal/vacuum coming undone. There is a hissing-type sound because the whole shaft turns whenever I turn the breaker bar in my attempts to loosen the shaft nut. I've tried using a crowbar to hold the gear (I forget which gear it is exactly, but it's underneath where the clutch hub and pressure plates and all that is normally seated, and it's what the secondary shaft is turning), while using the breaker bar and literally the tip of the crowbar broke off before the shaft nut loosened. I've used a good deal of "stuck bolt buster" spray on the nut but it hasn't helped yet.

Any thoughts? Thanks in advance!

Picture?

I'm at work now, but I'll try to post one later today or tomorrow

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