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What are those two green things on the shelf above your lathe ?
Might one or both of them be a "steady" ? (if so, it'll have three, threaded thingies pointing inwards.)
This can be fitted to the lathe bed and used to centralise the wayward end of the barstock while you centre-drill it.
I would be very wary of parting off anything that big in diameter.
Parting off is a dodgy process at the best of times, and worse if there is much backlash in the cross-slide screw.
The forces are huge, friction is high and, for a large diameter, the tool has to be long so its rigidity is compromised.
Even for smaller diameters, the tool needs to be set dead on centre height (or even slightly above, a heresy in normal circumstances) otherwise the tool will want to drag itself under the workpiece (made worse by cross-slide screw backlash).
Its useful for batch production but for one-offs I would just get the hacksaw out and take a deep breath.
But I would get as clever as I could in the turning operations so as to leave a smaller dia to saw if possible.
And/or I might part off to a shallow depth with a short, rigid tool, just to take some meat out of the work and also to guide the hacksaw.
For this dia, I would hacksaw in 4 or 5 different positions, to meet in the central bore.
Its almost impossible to leave a good face after parting off anyway, so you'll probably have to skim the back face afterwards.
And I would seek to arrange things so that I bored ALL of the bearing diameters/faces without disturbing the workpiece in the chuck, for concentricity.
The dodge with using the tailstock to help centralise is a nice little sign of intuitive skills btw.
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