No, there are three bed lengths each year and two difference series of beds.
Advance Design is just Chevrolet, GMC was called New Design. That's part of this because GMC made a long box 1/2-ton while Chevrolet never did. Your question is specific to Chevy but should be understood in that larger pricture.
'47 to '53 both manufacturers used one set of beds, starting in '54 both used another.
You need to know which truck you are dealing with to get the right dimension. For Chevrolet truck size and bed length go together. A 1/2-ton is always a short box, a 3/4-ton is always a long box, and a 1-ton if it has a box is a 9-foot box.
