I have a 1981 and I would think they would probably be the same. I'm not exactly sure of the tire size, but the rim is 15"x5" wheel. I just looked it up in my GM Shop Manual. The owners manual was no help.
The spare tire rim itself is a 15" x 5" wheel. The factory supplied spare tire mounted up under the rear. I've never thought of using a bigger wheel size or tire size.. so I'm not sure what would fit up under there and still allow the cover to close back up under the car.