Made a bunch more progress on this now that the choking smoke from wildfires has dissipated.
I had some limited success with disconnecting the lower end of the shocks, which would allow the swing arm to drop as far as possible, and allow the greatest latitude for the driveshaft to gain more clearance. For this to work, I had to use an automotive floor jack placed under the cross-bar in front of the center stand to raise the chassis so the swingarm could drop to its limit.
Unfortunately, even all this only netted about 1/8" or less between the flanges where you have to feed through the replacement through. This is an '82, so your experience may vary. I concluded that it was nuts to spend hours trying to feed the boot through, and taking the chance of damaging it in the process, when I could probably gain MUCH more clearance by loosening a few more bolts.
After some inspecting the mechanism, and more head-scratching, I decided to bite the bullet and remove the swingarm pivot so I could slide it back for more clearance. There are instructions on how to remove the swingarm pivot in the factory service manual, but I will be able to provide much simpler instructions after I do it myself. In a nutshell, you remove the three-domed plastic cover over the joint, spin off the the center nut with an impact wrench, remove the three retaining bolts using an ordinary socket, at which time you will have exposed the spacers on either side of the pivot point for removal.
The most obvious approach is to use a big slide-hammer and a small screw that fits the center hole on the spacer. The screw is only about the size of a 10-24 machine screw, so I was able to remove the one on the right side with several manly smacks of the slide hammer. There was significant corrosion around the spacer.
The left side was a whole different deal--for whatever reason, much more water seemed to have gotten into this joint. A bunch more manly smacks with the slide hammer produced only a snapped-off machine screw., fortunately above the surface of the spacer, so I should be able to grab it with a small pair of vice-grips and turn it out without further drama. I've soaked the area down with PBBlaster for the night and we'll see if it's any better tomorrow. The joint will definitely benefit from some aluminum anti-sieze compound for the sake of the poor bugger who inherits this beast after me!
I lost the sun after about 7:30 this evening, but I think the swing arm pivot has already moved significantly (i.e, 1/4 inch or more) rearward, which should make the boot installation a breeze and much more clearance should be available if needed.
That's it for now.