Just to be sure those lights are grounding properly (thru the alternator) and not as a result of the "fried" cluster, try disconnecting the small red wire at the alternator — making sure the free end doesn't touch anything.

Then turn the key ON and check the lights again. They should NOT be lit with that red D+ wire off the alternator. If they are lit, it must be due to something in the cluster.

EDIT: I presume that 12.8V reading you mentioned was at the battery terminals with the engine running?
If so, see what you read at the alternator B+ terminal (big red wire). If it's substantially higher, I'd start a new thread on that subject. Bad connections and/or wire corrosion could drop a volt or more between B+ and Battery +.

If not, if B+ is low too, I'd look at the alternator or brush/regulator, even tho' the bench check was OK. Sometimes those guys (primarily clerks) don't get tech things quite right it seems.
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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.