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Pallet wood is normally treated to preserve it. Some of the chemicals don't boil when the wood turns to charcoal and will reside in the final product. I was going to burn pallets to make charcoal for me raised bed and decided it was unsafe to grow vegetables with, never mind ingesting it like the author suggests.
If you're using two 2x4's side-on and one 4x4 in between doesn't that mean your bolts need to be at least 8"?