On a warm, drizzly morning in August, David Adickes shuffled slowly through his hangar-size studio just northeast of downtown Houston. Leaning on a walker, the artist navigated a labyrinth of half-finished sculptures, cluttered workbenches, and well-used machinery. Towering above it all was a gray, two-story-high concrete bust of President John F. Kennedy—hair carefully chiseled, jaw firm, eyes gazing confidently into the distance. Nearby, a studio assistant welded together the steel struts that form the statue’s substructure. The enormous figure appeared nearly complete except for a six-foot-long plaster necktie that leaned against Kennedy’s chest, waiting to be attached to his collar. Arrayed across a gravel lot outside the studio stood another 42 massive presidential heads—the series runs from George Washington through Barack Obama—which, to motorists taking an…