No.
The '41 thru '46 box is a little more narrow, but you can pull the ends off, trim each side down, and weld the ends back on and it looks pretty good.
That's what I did with my 46 1/2 ton.
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http://oldtruckstuff.com/mytrucks/rear_wood_on.gif[/img]
Well.. had my brother in law welder guy do it for me, anyway.