Poplar is usually the least expensive and since you're painting it the grain pattern doesn't matter either. I think the biggest issue will be the length you need to use so the bench doesn't sag in the middle. And the thickness of the slats might be a factor if the end pieces have a groove for them to go into.
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69 Z28 JL8 4 wheel disc brakes - being restored
09 Silverado Z71
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