historical correction R-bo, GM's fleet bed was not the first full width box of the industry, the
Ford full width Styleside bed started in 57 as a one-up on the GM Cameo [dressed up stepbed] .... Dodge also 'matched' the Cameo in 57 with the
Sweptside, pasting car quarters on a stepbed, and then in 59 came out with the
Sweptline full width bed - so ya might say the Cameo bed "evolved into the modern bed-type" but it was GM that did the copying with the wide Fleetside
also there was an Aussie Dodge in 52 that had a
wide smoothside bed - prolly called a "Ute", but predates anything US made of the fleet style

Bill