I thought holding the spacer with a pipe wrench only works for square bolts, not the round ones.
For round bolts I usually try to hold the bumper with a wrench and then tighten the nut; might take a 2nd person. Check it every few passes in the field until it won't tighten anymore. I take the wrenches out on the tractor with me. A good thick nut with more threads than a hardware store nut helps too.
As far as his disk having round bolt in square spacers, maybe bolts were replaced in the past.