I thought I'd post how it all turned out.
You all gave great advice. I didn't have any air tools so I purchased an inexpensive set from Home Depot. The impact wrench is only good for 300 ft-lbs, but I didn't need crazy power for this job; just wanted to start with a modest investment. While I realized my pancake compressor at 2.5 CFM @ 90 PSI wasn't up to the task for any real use, I figured it would give me a couple of shots which would be all I'd need to prove if the air tool would get the nut off or not.
I put some WD40 carefully on the threads only, avoiding lubricating the taper which wouldn't be helpful. I put a crow bar on the top of the joint to push it into the taper, then gave the the impact wrench a hit. Presto, the nut came off like nothing! Looking at the taper, the shoulder was not exposed, so I didn't need to use a washer to give me more compression of the taper. I just reversed the process and used the impact wrench to re-torque the nut. The taper seated nicely and everything was tight!
Took the tractor out for a test and she steers like a Porsche now. I wouldn't hesitate to take her right to 60 mph
On the downside, I put the chains on the front tires only to find the clearance between the tire and the vertical frame of the FEL is about 1/4 inch at full lock; there's no way the chains would clear. I should have checked that. Oh well, tirechain.com has a decent return policy.
All's well that ends well. Thanks all!