Narrowing an 8 1/4" rear end, how is it done?

Some yards don't show cars prior to '81 since the VIN has less than the 17 characters that their inventory system is set up for. That may be the case with at least as few of the yards that you are searching. It might be worth dropping in an taking a look if you are close enough.
Possibly. Although when I was hitting the one in Norcross, GA about 5 years ago for parts for my '93 Buick Roadmaster Estate Wagon, I noticed mostly what they had was later model stuff. They also have a really high turnover rate. If it sits too long with no activity or it's been picked clean, it's gone. This is probably very dependent on the value of scrap metal and wanting to make space for more desirable incoming desirable cars. I'm guessing the more rural and privately owned yards are where to find the really good stuff.