Thanks to all. You all gave good advice. In the end, I went to Tuffy Muffler and the guy there threw some tube of sealant at me and said "this is what we use."

Well, it was plain old lo-temp silicon. So I got some of the Ultra copper silicon, (700 degrees) dropped the header and used a file to smooth it best as possible (I could see from the soot on the gasket that the leak was on that side), put two circles of that silicon around the holes in the gasket and schlepped it together. Works fine. No more leak.
I used to be a perfectionist and I was sorely tempted to remove the header entirely and resurface it. But I've been listening to Car talk way too long so I won't worry about it till it leaks again.