I don't know what you mean by "scratch build" the mast. What is your media for the mast going to be? Brass, Plastic, and combination of medias, the kit mast with replacement parts? It can make a difference on how much work and details you need.
Also, Tamiya's 1/350 scale FLETCHER kit gives you a couple of options in building her ... to her June-August 1942 configuration (as completed and on shakedown) or to her August 1942 to July 1943 configuration (as she appeared in the Pacific) when she had the fantail twin 40-mm mount installed. Some equipment/antennas were added after FLETCHER got to the Pacific (mostly notably the BL "stovepipe" IFF antenna).
I can post images of the top of the mast showing the SC-1 and SG radar installations. But, those won't have dimensions.
Here is an image of FLETCHER's sister, JENKINS, with a better view of the masthead than I have for FLETCHER herself. But they both should be the same at this time.
Also, if you really are interested in drawing details on the mast construction and elsewhere on this class, the above mentioned DVD from Destroyer History Org., is a real help. I have attached a couple of examples from the BIW DVD below.
The foremast is actually a series of thick-walled pipe sections (looks like six) that reduce in diameter as they go up. I don't have specific dimensions for the sections. I can't locate the drawings for the mast on the DVD at this time (the bad part of the drawings on the DVD is that locating drawings by title/function isn't easy ... but going through the whole thing and making notes is a wonderful experience of seeing how a FLETCHER was made). Items like the radar including the motor assembly at the mast top, were "Government Furnished Equipment" (GFE). As such the builders didn't make detailed drawings of GFE like weapons and sensors, only those things they had to make to fit them.
These drawings come from Folder 5541-2, Tiff images #182 and #184 on the BIW FLETCHER Class Engineering Drawings DVD
