Updated: Nov 28, 2020
Dubbed "Streamliners," the vehicles looked much like moving vans of the period, sporting massive all-steel domed roofs. GM's Fisher Body Fleetwood plant in Detroit got the task of building the eight 23-foot Streamliners on 223-inch-wheelbase truck chassis powered by GMC-built straight-six gasoline engines (none of the sources we consulted identified the engines' displacement, though, in his history of GMC, Donald Meyer supposed it was 239 cubic inches).GM's designers specified that six of the Streamliners—in two groups of three—join via canvas awnings so that Parade of Progress visitors could tour the displays within. A seventh Streamliner incorporated a stage, which the massive 1,200-seat silver canvas tent enclosed for the 45-minute shows; and the eighth Streamliner provided support.
Sources: Hemmings, Reddit