THE EXPANSION OF JOHNSTON ISLAND IS IN STEP WITH DEVELOPMENTS IN THE ANP

Page 51 of the GAO Report on the ANP project shows that they were planning to do flight testing on an island in December of 1957. Johnston Island was expanded in 1958. The last 1964 expansion of Johnston Island occurred in step with when the SR-71 had become completely operational.

NOTE
1942 was the year that Enrico Fermi first proposed building a nuclear powered plane. 

SOURCES:
http://fas.org/nuke/space/anp-gao1963.pdf
https://en.wikipedia.org/wiki/Johnston_Atoll