Straightening stuff is kind of an art. I learned it at the first job shop I worked. Lots of dump truck hydraulic cylinder rams, and multi staged centrifugal pump shafts. It has to be done in stages, not attempting to get it all in one move. From the photo, I see 2 major bends, one fairly large in radius, one small.
If it were brought to me, I'd soak it in the oven, if it would fit, for an hours at 400-450 f, then hit the smaller of the bend points first, using a hydraulic press.....because of the slow, controlled pressure. I'd keep a scale, or some sort of straightedge real handy so as not to take too much time and lose the heat. A hardwood block might be a good idea for some things, but the direction of the bend makes the bottom, where the wood might go, present the teeth to the support. The teeth would have a tendency to sink in, and grab. You have to remember than when you flatten an arc, the piece grows in length. It must not be constrained from moving, and that might happen with a wood block.
I would bend a little, then release pressure to gage how much beyond straight it must be pushed, and allowed to spring back. If it is truly a hardened piece, it will spring back a great deal, and then you must apply a little more localized heat with a torch. It also helps sometimes to use a torch to keep from introducing more bends not centered on the apex of the starting bend. Smaller readii require the supports underneath to be relatively close together.
If all you have is an arbor press, you still can do it, but I'd recommend some support under the center of the bend a little taller than the end supports. This will limit the overtravel of the ram, which might happen under just hand pressure on the press handle. You might overbend without it.
On the cheap, quick and dirty (CQD method), a machine vise that grabs the straight section, and a strong arm is applied to the rest of it, and pulls or pushes against the bend. I prefer to pull, and I can catch myself on one leg easier that way if something breaks. If more control or leverage is needed, a pipe can be slipped over the piece.
That reminds me, I straightened hundreds of shafts from PW drives for a local boat shop. I never did see how they got bent, but they brought them by the dozens to be straightened. Usually got them with 0.005 TIR anywhere along the length, which was 3-4 feet.
On the lost temper (pun intended) issue, unless you heat it to color, which isn't necessary here, that won't be a problem.
