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they are all same size just different lengths depending on which hole they go to in the bed, and the nuts are 3/4. can't remember off hand what the lengths are for the different holes in the bed/frame.
The bolt size is 1/2"-13 X 3 3/4" NC. Most people use regular 1/2" carraige bolts but these aren't correct. The heads and square shank are a lot smaller than the OEM bolts.
Dennis Carpenter makes the correct bolt and sells them as a kit for around $25.00. The bolts are dead on but the nuts aren't. OEM nuts are crimp-lok lock nuts, Carpenter's kit has regular nuts and lock washers.
Here's a picture of one of Carpenter's bolts.