I would go with SPF; strength is not the prime concern; twisting is.

Years ago when I was building houses, I had a visiting lumber salesman ask for my materials list, and he gave me a price that looked very good. What I failed to notice was that he had substituted 5/8 SYP for sun-flooring. It went down great, and stayed that was for maybe a week. Then the humidity got to it, and it warped, twisted and moved all over the place. First, last, and only time I ever used SYP sheathing. Yes, it is strong, but it is also strong-willed, and has a mind of its own.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags