Drilled holes to mound the bending brake bottom plate to the bench. Because of the way my bench is constucted, I drilled through the benchtop into the 1x4 vertical frame and mounted the plate with countersunk deck screws.
The top plate clamps to the bottom plate with 1/4-28 screws threaded into the bottom plate. Hopefully with the small bends I need to do, the bottom plate bench attachment will be adequate. In Dave Clay's method, the 3/8 clamping bolts help secure the whole assembly to the bench.
Deburred all and touched up the primer. Tomorrow I should be ready to rivet the hinge and attach the assembly to the bench.