Yow, that's a helluva mess don't do it that way, pa-lease.
Do as Mike suggests and move that wall out a little with new studs sistered to the old or firring strips attached to the face of the studs. THEN you can bring your wallboard down to the top of the flange.
Use a few shims and screws to "set" the tub but don't worry about it after that. Once the tub is trapped in the hole it isn't going anywhere.
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