I was involved in something similar to this a few years back, they used a three part die with upper follow or squeeze rollers..
the male top push die was relieved on the sides and the follow rollers would over-bend the sides slightly to achieve a straight side after spring back..
they also had a lower die follower that applied pressure with a hydraulic accumulator to achieve a flat bottom surface as without it there was tendancy to round the bottom flat while bending..
that die set _mostly- worked ok with the exception that occasionally it would gall a little and then pull one side resulting in unequal finished arm length..
afterwards they went to a hossfeld style manual bender with a shop made die, and finished it with a hammer die that was basically two notches at a slight inward angle that compensated for the springback, and a flat top surface to get the proper flat on the center, basically push it down into the notches and smack it with a hammer, surprisingly effective...
enjoy..