Riff raff mades a good suggestion... I was going to say get two shorter bolts and then home depot sells internally threaded spacers that are meant for joining two bolts or threaded rods together. I would try to find a way to secure one of those inside the super bumper and Then you only have to use small bolts on each side. It doesn't matter if they connect because the strength of your hitchpin comes from the shear strength of the material at the joint of the hitch and whatever you are using the hitch for

I would try to get a pin that is as strong as possible, your super bumper will probably just push into hitch instead of compressing if the pin breaks, which would focus all of the energy of the crash on your gas tank or rear frame (it's been so long I forget what is behind the hitch)