When figuring overall legnth you figure not only BBC+box legnth, but also the space between the cab and box(for the flex boot) and the rear bumper and then round up to the next even foot.
So, a BBC of 112"(9'4")+ a 30' box + a 4" flex boot and a 6" bumper get's you to 40'2". Since it's over 40', you end up at 41'. Add 8" for te 120"BBC and you end up with 40'10" which, again, puts you at 41'.
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