i'm thinkin the replacement box on my truck is a 66. mainly the board width were different but i made my own boards using plastic lumber so i had to drill the holes into the cross struts for the different spacing. mine came from bills trucks.
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There are 106 miles to Chicago. We have a full tank of gas, half a pack of cigarettes, it's dark, and we're wearing sunglasses.
The big difference between years is the wood - number of boards etc....other than that they are basically the same. I forget what year they tailgate changed slightly also - just a cosmetic change to the embossed chevrolet.