Buick used the 9790377/8 spacers on the '68-72 GS400 and GS455 but not the GS350. The spacers limit the amount of rear-end squat during hard acceleration. If your car still has the 350 motor, it should do fine without the spacers.
The only other parts book listings for the 9790377/8 spacers appear for '68 Chevelle wagons and El Camino. Buick used similar spacers in the regular A body wagons '68-72 under part numbers 9793636/7. SportWagons did not use the spacers. The 9793636/7 nos eventually superseded the other numbers and covered the applications in most all GM '64-72 A body cars having rear axle bumper spacers. That gives some good leads in finding spacers in salvage yards.
What has been, can be again. (Bob Wills, 1942)