O1 would be good, or 4340. The latter wouldn't need heat treated, the O1 should be hardened and drawn to about RC35. CRS would be wasted, it would too easily bend.
The B & S clamps I have are very similar. I rarely use the long end, clamp just beyond the middle screw.
I'd use 1/4 28 screws, too, easier to snug up without overtightening.