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Thanks very much, gents,

Looks like this one's out. I've replaced the shims - not that it is immensely easier to do with the camshaft out of the way, I wouldn't be able to get the shims out with the cam in if my life depended on it.

Found a bunch about .30 mm under the right size (valve lash about 0.6+ mm). One I had to leave at 0.432mm cold, one ended up a bit too tight at 0.297mm, the rest was okay. The head I pulled off at the junk yard had the shims of almost the same size - either 3.95 or 4.00 mm.
Engine runs by far quieter (before, when my son was coming home late, I'd hear it a block away); seems to have a bit more power, but it may be purely subjective. There's a persistent tapping sound, however, that did not go away. I hope it's not a bad rod bearing - in my experience, there's about a 1000mi from audible rod knock to oil pressure light on, that one's been around longer.

One thing I should say - after american V8s, and, especially, after Land Rover V8s (Buick 215 derivative), this engine is such a darling to work on...