I'll second the suggestions to keep it freshwater if you want to keep your piece of wood. I have three beautiful pieces of wood in my brackish water tank with my two GSPs, but they are going to have to be taken out (I'm planning to switch to full marine by the time I move cities in a year). Every week when I clean the tank and rinse the wood there is definitely a lot of "brown" rinsing off of the wood, and my salinity right now is only at 1.008.
However, I do want to say that a single fish in the tank may seem like a "waste" of space (the local fish store told me it was quote "ridiculous" to need 75 gallons for my two inch-long GSPs, groan) but it most certainly isn't! They are so active and curious and inquisitive, they really do take up the entire tank. My fiance had the same questions your wife did, about their needing such a large tank. Now though, he's saying to me, "well, when they get big they'll need lots more swimming room so we'll get them 90 gallons when we move and use this tank as a freshwater planted tank."
(I said sure, so long as you help me clean them both
). It's an amazing treat to have a space where your fish seem to have room to explore and swim and behave in a way that is more similar to the way they'd behave in the wild (more so than in a smaller tank, anyway).