That's a good plate design,I would use the same amount of holes only around 5/16 dia. A plate w/9/16 hole's might allow the surport plate to fracture sooner(plus larger holes mean more welds and more heat it the weld zone..A smaller hole will allow the plate to provide more of the surport needed.Keeping the end's of plate angled will not let the frame rail shear as easy.
