In about eight seconds, I went from ... "what's different about the '73-up F-series Flareside body?" to Fordification.net and the link below.
If you wanted to be a stickler for your AMT '75 and up Effies, you could build or modify the Flareside box to suit these dimensions. Note the illustration shows the standard WB municipal-like Flareside; a shorter 116" (?) WB version was featured in the advertising and owns a place in the heart.