I don't know whether this has been covered before (I searched «"spark plug tube seal" "O-ring"» and didn't find anything), but in buying my million-dollar pile of tune-up parts, I discovered that slant-6 spark plug seals are now unavailable.
Some places list them, but show them Out of Stock, while others no longer show them at all.
None of this is terribly conducive to actually laying hands on a set.
With the need for an alternative as I clean up the new dart, I gave it a thought and decided that there is very little difference between the spark plug tube seals and regular O-rings, aside from the square vs round cross-section.
The seals on my motor are 1.5" OD / 1.25" ID, making them 1/8" thick.
Coincidentally, the Size-218 O-ring has these exact dimensions.
(Available at your local hardware store as Hillman 56051-B for 69¢ each, which adds up to about 4 bucks, less than the "real" seals cost).
On installation, they seem to fit the same (I have no photos because they are indistinguishable visually), and the sleeve rests in the same spot against the head when the plug is tightened.
There is salt on the roads here now, so the car won't be driven until the Spring, but I'll post again if they leak or otherwise don't work.
In the photos, the factory seal looks thicker and tapered, but, as far as I know, that's just the set it took after 60 years in the head.
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