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Odd that there's not that info. in the tractor WSM, I had a look too. Perhaps someone has a standalone V4000 manual (V4000-B manual is Kubota code no. 07909-70103) or product service bulletin with that info... .
Subject to others' better info or ideas, because the stud is into cast iron (?) I'd use the torque Kubota references for the lowest grade M8 bolt - 18-20 N.m. and check for leakage.