As promised, here are the pics:
Trying out the 71 Fury hubcaps EricW gave me. Still seeing if i like them or not:
I spy, with my little eye, an 8 3/4!
Yes, there is the removable (and heavy) third member:
The axle is happy in its new home:
But my rear leaf springs are getting worn out:
And here's the cause of this whole adventure, three teeth sheared off of each of the spider gears!
No wonder the old 7 1/4 rear axle was making banging noises.
So now I need to take a couple days to recover, and then it is on to swapping out the lean burn intake, carb, and ignition to an Edelbrock SP2P intake, Holley 4360 carb, and HEI ignition. I also need to get the correct speedo gear in the tranny since my speedometer now reads much too fast.
Late on I will replace the whole exhaust line with one that has no catalytics (instead of three) and a free flowing Walker Quiet-Flo muffler, and maybe a resonator. Boy am I glad this chore is done and everything worked.
I took the car for a shakedown cruise on some errands, and everything appears to be working well. Brakes are good, no inapprpriate noises or grinding coming from the back, thr car tracks straight and smooth, no tire rubbing over bumps or around corners. For the brakes, I used the rubber line, hard lines, backing plates, and all brake hardware off of the old 7 1/4. It all bolted right on and the rubber line was even long enough to make it to the new location of the junction block on the axle. All in all, this wasn't a very difficult swap. The trickiest part was drilling out the mounting pads, but lots and lots of cutting oil made that not as bad as it could have been.
However, if I could go back and do it all over again, I think I would have rather taken up Jason White on his offer of the rear leaf springs, sway bar, and 8 1/4 axle out of a Diplomat cop car. Cost would have been less, installation would have been easier, and it would have gone much faster. But I didn't and it all worked out okay. Now I need to go take some painkillers and muscle relaxants before my back spasms permanently.
Thanks to everyone who put up with my endless questions and offered advice and assistance.