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

I think things have changed at Pull-A-Part. I searched for any Plymouth model between '65 and '76 in Georgia, Tennessee, Alabama, and South Carolina, and came up with 2 pre-1990 vehicles. Technically, the one you mentioned is in Augusta, GA. It backs up to the west side of the river. The east side of the river is SC. But I knew what you meant :)
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.