Thanks for all the suggestions. I'll wrap this up.
1. I was replacing the blades because they were heavily gouged from hitting too many rocks, and were out of balance.
2. The diagram I posted showed a key, but the diagram was from a manual covering 5 different sizes of the Bush Hog Squealer. The dealer told me the key is not used in the two smaller size Squealers, so the diagram is not accurate for the 142 or 148 models.
3. I didn't have ready access to a torch, so I used a grinder with a Lenox MetalMax cutoff wheel. I needed to remove the safety shield on the grinder to make the flush cut, so the Lenox was well worth the price both for cutting ability and greater resistance to shattering.
4. I made one plunge cut directly across the top center of the bolt, down through the blade, until the blade fell away. Then I made a flush cut through the bolt where the blade had been. That left about 1/16" of the bolt exposed, including the shoulder that had been holding the blade centered on the bolt. I used a metal grinding disc to grind off the remaining shoulder flush with the stump jumper until the shoulder was gone and I could use a punch to push the bolt out. Two cuts, one grind. About 90 minutes each for two bolts.
This would not work for regular blade replacements, as the technique inevitably grinds away small portions of the stump jumper.
I want a cutoff torch for Christmas.