I don't think that the 947' length was what he was originally aiming for... and at 33.75 inches the ship would have been 938.25'. So I think when the drawings we saw (with all the dimensions on it)
was drawn up for series production (1966), Jefferies was most likely aware that the components were placed differently on the models than on his original plans. Had the original plans been slightly over 34 inches in length, then I would have thought that it was part of his original intent. As it stands, only the component sizes reflect his original scaling from 1964, the overall length was most likely a casualty of the rush to finish the models for the first pilot.
At the time (fall of 1964), I think the only thing that really mattered was that the bridge dome diameter work with the bridge dome set plans (the bridge set construction was started after the 33 inch model was started, and filming of the bridge set scenes were finished before the 11 foot model was started).
The one aspect of his original design he seemed to not want to let go of was how far apart the nacelles were from each other... either he didn't know that they were moved closer together on both models or he didn't care for the fact that they were moved together.
I personally think they look better closer together.
Of course anyone wanting exactly
947 feet can just move the nacelles back slightly on their supports... I doubt anyone would notice.