A very interesting article, to me anyway....
www.customcarchronicle.com: History of the Early Custom Car by Rik Hoving
He starts with the 1933 Cadillac Aerodynamic V16 Coupé displayed in the GM pavilion at the 1933-34 “Century of Progress” Chicago Worlds Fair.
Declares this the first important custom car, the So-California Plating Co. 1935 Ford designed by George DuVall and created by DuVall, Kurtis Chad Schultz, George Thomas Top Shop and Jimmy Summers
This was a four-door sedan stretched a foot and converted to a sedan delivery. A details I hadn't noticed is the Woodlite headlights, normally in freestanding chrome nacelles, embedded into the fenders. The original high-tech lgihting.
This picture captures it's importance, placing it between the 1932 Ford shop truck and a typical car of the era. Lots of interesting stuff as well, here's Gordon Beurig's first car -- A 1930 Model A Ford. It ahd a step-down frame like a later Hudson.