Space saver spare tire in an A body?!?

You are corrct sir!

I saw an article once where they mentioned that with different size left and right rear tires, you can burn out the limited slip unit in as little as 20 minutes of highway driving.

I think it was referring to large tires on a 4WD with something like a 3-4 inch height difference. You still do not want to take a chance with any height difference.

For a rear flat, you could always put the spare on the front and move the front tire to the rear to replace the flat. It may be worth the extra 15 minutes.