Just to be clear, I think he meant weld the holes in the bumper shut and then drill new ones in the bumper that will line up with the existing frame holes.
Yes, this is what I meant. This method would allow you to use the existing threaded holes in the frame.
I guess I could drill through and put bolts all the way through the frame rail. Good idea.
But I also think this would work if you can manage to reach the outside of the frame rail and access the hardware going through the frame for a through bolt configuration.
Either way this sucks that your bumper doesn't fit. It is BS that 4x4 labs told you to wallow out the holes. If you do that and get a good hit to your rear end chances are the bumper will slide forward and damage the fenders. It might even cause the hardware to shear and lead to increased damage instead of protecting the rig like it was designed.
Personally I would not be happy if I were in your shoes. I would likely be asking for a refund and looking at a delta vs or slee bumper instead.