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Greg,
Your 15' should have wood under the deck in all flat spots. There is a wood locating diagram for your boat on this website. For all screws on your Whaler, I would recomend stainless. On my restoration, I used 100% silicon for all screw placment. I pre-drilled where a screw would go, then inject the hole with silicon. Then I would verify I had enough silicon to complete fill the finish washer. As for your idea to remove your seat back. I believe you would be lossing valuable storage. You can look at my personal page to see the Dri-Dec and a 12 gal fuel tank. Good Luck, Paul