One 50mm hole through the centre of each 20mm thick plate. A single M48 threaded rod through all plates then support the plates on nuts and washers spaced 120mm on the rod. Simple.
A very large bending moment would be my guess!
The formulae in 7G allow you to calculate the compressive stress at the torque shoulder due to make-up torque. So I suppose you could use the ol' Euler-Bernoulli bending calc, treat the box and pin like one continuous pipe, and work out the bending...