I've made spreaders from spruce, fir, and mahogany. Spruce is the old standby, I believe, while Douglas Fir may be cheaper for clear, straight grain. I think Hickory and Walnut would be poor choices, as they are both quite heavy and are not particularly rot-resistant. not to mention that Walnut was traditionally a bad-luck wood to have aboard a vessel (so I have heard). you might post this over on the woodenboat forum to see what opinions they have (and they do have some strong opinions on that forum!) :-)