Well, I'm back again and still no luck. The impact wrench I bought, even though it can put out 1000 ft # didn't do the job. And that's because the spindle starts to turn the same as I had indicated in my original post when I applied a lot of torque using a big breaker bar with a pipe over the handle to give me even more leverage. I had hoped that the hammering action of the impact wrench would make a difference, but it didn't. Tightening up the belt that drives the blade pulleys didn't help much either. I haven't tried the acetylene torch idea yet because I don't have one. But when I was in Home Depot the other day in the plumbing section, I noticed that in addition to tanks of propane, they also had something called MAP/Pro premium torch fuel. Same price as the propane tank, but a warning on the MAP/Pro can stated "Use only with torches designed for MAP/Pro gas. I asked the HD guy about the difference from propane, and he said it heated the copper fittings faster and hotter, but had no other specific information. Does anyone know if this would be hot enough for me to have success with what Piston Slapper suggested?
I haven't tried grinding the head off yet, because if the remaining threaded section of the bold doesn't come out (and I would have nothing to grab hold of after the grinding), then I'd have to remove and replace the whole spindle. But now I know that neither a cheater bar nor an impact wrench can solve the problem because the spindle turns before the needed torque to loosen the bolt is achieved. Any other suggestions?