The holes in the wheels didn't seem unusually shaped, the threads on the inside looked OK as well. But since you mentioned it, I will pay particular attention to that when I get the new bolts. If they don't look properly round, I guess I'm in for a BIG expense replacing the rim! And they would have to come get it, no way I'm going to try to get that big, loaded tire off and move it even 1". Wouldn't fit in my Prius anyway I don't think.
I find it odd that such a big tire is held on with 6 not all that long flat head bolts. Not even tapered like the nuts for the lugs on my car to help center them and give a surface for the rim to "sit" on. Nope, just bolt threads.
Had a bit of a DOH! thinking about it. I had gotten out the 20 ton hydraulic jack to lift up the tractor to tighten, as much as was reasonable with bolts that have mostly flatted out threads, the remaining bolts. Bet the backhoe stabilizers would have done the job just as well if not better. And no looking around for the right size pieces of 2x12 to put under the jack.