After some research, I found I could not only replace the missing cushions, but upgrade the stock 5/8" soft rubber cushions as well, by installing 7/8" firm rubber cushions. So Wednesday I placed an order with the local Chevy dealer.
The parts arrived today, so I set about trying to install them. After struggling with trying to remove the first bolt, I finally realized because I was working upside down, I needed to turn the wrench the OTHER way. D'OH! After that, it was pretty easy, though rather tiring, since I had to work laying on the ground under the car.
End result, 6 metal washers and 6 nasty old rubber cushions lying on the pavement, and 12 fresh new cushions installed. And the car handles SO much better. Almost no body roll at all, steering response is instant, and the ride seems a bit smoother as well.