Well, the Dodge Monaco O-rings are a perfect fit (of course) but I still have a vacuum leak at cylinder #4.
Although the package says "Not for use with gasoline," I decided to use some leftover RTV to bolster the O-rings and bolted everything back down tonight. It didn't look like much when I squeezed it in, but when I tightened the manifold down it squished out and looks rather horrible. I'll leave it to cure overnight and see what the morning brings.
I just can't imagine why the O-rings that were installed were problem-free for hundreds of miles, and then when the car sat for three weeks they had somehow shrank. I don't get it.