To assemble the large Saturns, NASA needed a plant, preferably one already built. The Michoud facility, close to New Orleans, suited the requirements. A survey of government-owned war surplus plants revealed one near St. Louis and another (named Michoud) near New Orleans that were suitable for building the huge boosters. But the Mississippi River around St. Louis often froze over during the winter months. So Michoud, with a mammoth building that contained 0.17 square kilometers under one roof as part of a 3.5-square-kilometer complex along the water's edge, was selected on 7 September 1961.* Designed as a shipyard, it had become a cargo aircraft factory in 1943 and a tank engine plant during the Korean conflict. Here the Chrysler Corporation and The Boeing Company would construct the first stages of the Saturn C-1 and, later, of the C-3, C-4, or C-5 (or whatever model was chosen).
SOURCE: SP-4205 Chariots for Apollo: A History of Manned Lunar Spacecraft (NASA, 1979) By Courtney G Brooks, James M. Grimwood, Loyd S. Swenson