I have noticed that as I drive on wavy roads, the rear suspension seems to bottom out. I recently had the rear shock absorbers replaced. Do the rear coil springs need to be replaced? Are HD coil springs available for this model

 

Are you sure the correct shocks were installed?

 

The shocks installed were ordered from Rock Auto Monroe 5820. The problem with the bottoming out existing even with the previous shocks,

 

From looking at your pictures in the thread link below, I'd say your springs are fine. Usually when springs on these cars are bad the rear of the car will sag. Look at other parts of the suspension and see if any bushings are worn out.
Owner of a 1968 98 convt - ClassicOldsmobile.com

 

Rock Auto has these Husky Springs RV503 that have variable rate coil springs.