Those are not threaded holes that can be drilled out and retapped. They are industrial-strength threaded inserts.
The correct repair is to remove them and to install new inserts and new bolts.
They are a little pricy, but trust me you DO want the OEM parts if you cen get them. If not, you will have to consider the parts, tools and techniques available to you there to correct.
If your original inserts are serviceable but usable, an expedient repair is to lightly tack weld them to the frame so that they don't spin when you tighten the bolts. This way you can still remove the insert should you decide to replace it.
Good luck, and let us know how you make out.