Clearly nobody is actually opening their eyes and reading the graph.
It says "Wheel torque ft lb", NOT flywheel. Now considering the gearbox is a torque multiplier and it doesnt mention Engine torque, why is it so silly?
Im guessing its just some funky old RR that calculates the power slightly differently that modern day stuff. You clearly arent going to peak at 160 odd bhp with a Fly torque of 350 ft lb. (duhhh)
Divide the 350ft lb's through the ratio of the gear it ran in, then by the final drive, and maybe a few other numbers and its pretty obvious you will engine up with the torque figure everyone is expecting.