I’m in exactly the same boat as I’m also using reclaimed shelves, although mine already include edge joints. I will joint them to twelve inches wide once I’ve planed off the polyurethane (man that stuff blunts planes quickly!).
If I had boards as nice as yours and was not planning to paint (I am though), I’d aim to keep those panels whole on the sides and back.
I really don’t see why you can’t place the saw kerf exactly on your glue line if you accept the need to plane down a little more on both sides than with a full width board. Just use a marking gauge to define the horizontals. Caveat is I’ve never done it and am a beginner 🙂
Keep us updated! Cheers, Andy