Would not be concerned so much about the bumper, but more on sand cast front axle, since the cantilever weight combines additional shear stress to the bending stress already developed from the basic tractor weight.
However, addressing the bumper which is the current subject and not the axle,..
On dynamic loading, DOT uses a dynamic load factor = 3X for highway.
For barn yard and dirt field work could reduce dynamic load factor to 2X.
The static stress on the bumper bar at the bend where it bolts to the axle ~ 15 ksi compared to 36 ksi allowable for hot rolled steel, based on a 400# weight.
Applying a 2X dynamic load factor places the stress = 30 ksi, and safety margin = 16% ,which is safe enough.
The attachment bolts are loaded by a "heel-toe" arrangement.
With a 400# cantilever weight, there is plenty of safety margin on the 5/8" bolts, even using grade 2.