[OP] Done, both sides. For the record:
- I cut the bolt/sleeve, above and below the bushing, as close as I could to the frame bracket surfaces. I should have done that right from the start.
- I had to cut the nut cages off - they didn't just pop off. I reached in with a hacksaw blade to cut the front and back sides free from the frame surface. It was still welded on the side behind the nut, but I was now able to pry/wiggle it, and break that free.
- I was able to reach the passenger's side cage from under the car. The driver's side was much more obstructed - I couldn't see a way to even touch it from below. So, I removed the steering rack and the control arm bushing clamps. That let me reach the cage and nut from the wheel well. I haven't read anyone saying to do that, so I might be missing something; but, that's how it seemed.
- To remove the bushings, I cut the rubber out with a coping saw, and then used a hacksaw to (carefully) cut the outer metal ring. After that, it just popped out.
- I was able to press the bushings in, just using the (new) frame bolt, some pipe (2" on the bushing), and a couple of heavy-ish plates. It didn't go easily, but I've read that you need a 20-ton press, and I didn't.
- I got new bolts from Chevy. Matching all-metal lock nuts at Amazon