Well, anneal it unless it's a Reeves Goehring guard (already soft). You can do the work over the horn of an anvil but you'd need to work alternatively from one side then the other of the anvil. Otherwise, because the horn is cone-shaped, as you draw it out it will stretch more on one side than the other and curve. Another option is to draw it out on any nice round solid steel stock that you can clamp in a big vise. Say something about 1.5" in diameter. Remember, any flaws or roughness in your hammer or anvil horn or cyclinder will show on the work and have to be filed out later. Might be a good time to smooth things up.
First I'd roughly file out any roughness or casting flaws so you don't have problems later, having to file out hammered-in flaws after the bow is thin. Then I'd hold the concave inner surface of the bow on the anvil horn or cylinder and then work the convex outer surface with my favorite forging hammer- a 2.5 pound, smooth faced cross peen. I'd start at the middle, work toward one end of the bow, then from the middle to the other end of the bow, then straighten everything, restore the right curvature, then re-anneal. I'd pretty much work on the centerline and not hammer near the edges. That would just make the guard curve. Repeat as needed. In the end, I use a wooden form to get a nice smooth curve to the bow. It's not easy for me to get all the little waves and flat spots out freehand.
A 10-20% increase in length is achievable. More than that is a "stretch". Leave more thickness than you think you need so you can still get a nice lens-shaped cross section to the bow.
