How do you clean your slant six plugs, or do you even clean them at all?

Five of your plugs don't look fouled to me—not at all. They look fantastic; they speak of damn-near-perfect fuel mixture and excellent oil control. The uppermost plug is slightly dirtier, but still has trouble meeting any real definition of "fouled". There's no crusty, gummy, or flaky buildup, and nothing bridging the gap.

If the engine idled a lot better with new plugs, I'd say leave 'em in awhile and see how it goes. You should be able to get at least 20k miles out of a set of good-quality plugs—one of the advantages of today's much cleaner gasoline versus the dirty gunk that was available when these cars were new.

Also quit buying "tools" from Harbor Freight—it's easier and more efficient to just set a pile of cash on the street and light it on fire.

Also, back in the "good old days" when service stations had sandblaster-type spark plug cleaners, a whole hell of a lot of sand stayed in the crevices of the spark plug, and got released directly into the engine after the plugs were reinstalled.