Those three holes on each side were for brackets to lock a 2 pt hitch in a solid position. The solid position was needed for planters and other equipment that was rear heavy so the hitch would not rear up. The top pins of the hitch were also put in the lock position for mounted corn pickers so the rock shaft arms could be used to lift and lower the picker on the 234 model corn picker.
That would be a good place to mount the backhoe.
You might be able to find some of those brackets to make things easier.