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Ah, for some reason I totally overlooked the mid lengths you listed.

Stock manifolds don't give much room for timing to be added up top. Can't say I'm shocked.

Try a slightly colder plug.

Are you sure it isn't just a bad tank of gas? I've had cars with 93 octane from QuikTrip that knock their ass off and timing ends up about 10* lower than it should. As soon as they get 93 octane...