The NMM did receive the main Denny plans collection and a catalogue was compiled by David Lyon when he worked at the museum back in the 1970s which was published in four volumes as 'The Denny List'. I have three of these but can't look at them at the moment as I am not in the UK. I bought the plans for ms Shanklin from the NMM.
As Mudway says, I think that Glasgow University may also hold drawings as I seem to recall somebody mentioning that they were cheaper!
Colin