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As luck would have it, I just pulled mine a couple of weeks ago in preparation for pulling my (long) bed off. They appear to have been unmolested. There are six carriage bolts: four are 7/16 (diameter) x 14 (threads/inch) x 2 inches long. The other two have the same dimensions but are 2 1/2 inches long and they go at the front of the bed (closest to the cab).
John's F-Fun Hundreds in Pomona, California (recently moved from Anaheim) shows them on page 70 of their catalog, viewable on line, saying they are the same from '61-'72, but at a high price which is uncharacteristic for them - they are usually pretty reasonable. LMC Truck's current catalog, page 24, shows Styleside Bed to frame bolts, '67-'72 at a much more reasonable price. I would call LMC and ask if they will also work for your '66. Good luck.
Thanks super saber I have the lmc mag. Think I will go with them I will give them a ring tomorrow to see if they will work on the my 66 I didn't even think of looking in their thanks again low66