We have built a lot of that style beds. Really depends on weather your using green or dry wood.
We use four concrete blocks for a 4'x8' pallet style beds. We had one bed this spring that wood had dried some and didn't sink with four blocks and had to add a fifth block to it. Have also used four blocks on 5x5 and 4x6 beds. But we only use green sawmill wood. If your using pieces from dried pallets it may take more weight.
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2018 Crappie Masters Kentucky/Tennessee State Champion