My strong suggestion is to believe the Campbell plans, unless there's some strong reason to do otherwise. He was the naval architect in charge of the restoration of the ship in the late fifties and early sixties; I strongly suspect he actually measured the planks.
The verbiage in the book (at least the edition I have) is: "Planks average 24'0" long. Teak, yellow pine (British), white pine (American) thickness 3" teack, 3 1/2" other[.] Width 5" to 6" normal, or 4"-8", teak 6" 8"[.] Margins about 9" teak or greenheart[.]" That's a little bit cryptic, but I don't think it really contradicts the notes he wrote on the plans.
Be careful with photos. The planking of the Cutty Sark's deck has been replaced several times, and it's not safe to assume that the widths of the planks have remained constant. The first time I saw her, in 1978, the carpenters were replanking the forecastle deck. One of them explained to me that the main deck was covered with some sort of tropical hardwood (not teak), which was stuck down on top of the original planks with some sort of adhesive. (That explains why no boltheads or bungs were visible.) Those modern planks have now been removed. I assume the original planks (or some of them, at any rate) were still there; I'm sure the conservators will replace them with some authentic-looking material.
The ship's website contains lots of information about the research done during the current restoration. There may well be something there about the plank dimensions.
Remember that, on the scale of the big Revell kit, the difference between a 5" plank and a 6" one is pretty small - 1/96 of an inch. (That's slightly over 0.010".) In the U.S. (I can't speak for other countries), pre-milled "stripwood" can be found pretty easily in 1/16" widths. That's 6" on the scale - mighty close. To get 5" wide planks you'd have to mill them yourself - not a major problem if you've got a miniature table saw, but extremely difficult otherwise.
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