Jason, Don't take offense at Ross. I think he was just pulling your leg.
Sitka Spruce, at least the long grain variety commonly used in spar making, is extremely hard to find even if you could afford it; so fir is probably an acceptable substitute. One thing to know however is that the best looking fir, that is; the clearest looking and straightest grain fir comes from old growth timber, and most that that suddenly came on the market a number of years ago, was recovered from old growth fir that had been downed, in the many millions of board feet, from the old growth forests around Mt. St. Helen, when the volcano erupted. For some strange reason, that no one has been able to explain to me, the word had spread that that fir was subject to premature rotting. I have no personal experience to support that claim, but thought you should know about it anyway. Maybe someone else could comment on it.But to get to your project, Frames are no longer steam bent, tillers are seldom shaped from one piece, and most beams are no longer sawn from one piece. They are all laminated. Laminates are stronger and more predictable in failure, better looking, easier to shape, are more rot, split, and warp resistant, and are usually cheaper. Why not look into that? Good Luck Joe S