@rwm ,
Using the pins gives you very limited contact area with the nut, which means stress concentration and possible deformation.
I don't know the thread size or how much torque you expect to to apply, but if it's substantial, I suggest using a different design approach.
I'm thinking of using rectangular stock to essentially fill the grooves in the nut.
Rough order of operations:
1) Drill a clearance hole for the fastener size (at center) deeper than the finished tool length.
2) Turn and face the 25° socket end.
3) Mill three slots (at the 25°), radially as needed (equally spaced?), the width and depth of the slots in the nut.
4) Saw/part off the tool to length.
5) Make 3 drive lugs to fill the tool slots and fully engage the nut slots.
6) Install a nut into the tool, with the slots aligned, using a screw through the center hole.
7) Position and weld each drive lug in a slot.
8) Finish machine as required.
Using that approach, the interior of the tool is finished after welding so you don't have to re-machine the 25° (interrupted cut).
Regarding material choice and heat treating, it depends on the magnitude of the forces involves and how long you want the tool to last.
Making all parts from the same heat treatable material will allow you to harden and temper after welding. A2 and O1 are easily available and can be flame hardened and tempered (if you don't have an oven).
Also, rather than mill a hex on the back of tool, why not weld a surplus socket there to get a square drive? Attach the socket after heat treating the tool. The heat affected zone won't be a problem to the business end of the tool.
I hope this is helpful.