Try these suppliers. Measure yours to be certain you get the right ones....a GS uses wider radiator side tanks than most Special/Skylark applications. They're expensive, maybe you can find a Chevelle/GTO product to work, or just make something. CARS http://www.oldbuickparts.com/ Steele Rubber: https://www.steelerubber.com/
The GTO ones look like the insert tabs are not correct, as I remember the 66 ones are 45 degree angle.
A 66 Olds and the 66 GTO seem to be coming off the u-shaped rubber at about 90 degrees. It just goes to show you how independent the GM divisions were back then.