Paul,
You're right. We generally don't think about the spare tire and what condition it's in, etc. As the old adage goes, out of sight, out of mind.
I'm thinking possibly a space saver from a Monte SS, being that the spare would be closer in size to the tires on the car....like the standard 215/65 x 15 on an SS or GN. However, a Monte Carlo, or other G-body car would not have to contend with a narrower/smaller tire. Unlike a wagon, there's no well to contend with.
Good idea. For my Monte I was able to just stick a full size spare in the wheelwell, if I recall. Been awhile, I have stock spares in them now, but a bigger wheel/tire would be an issue, but a 15" up to 235 I want to say, would fit snugly.