Ouch, it does look like a crack. Your first pic was pretty fuzzy. I assume you had oil come out of the hole for the bolt that went in vertically. Did that bolt have a flat boss where the hole was or just the rough casting. If the latter, that would kind of reinforce my idea that someone drilled and tapped for the bracket for some reason. It almost looks like someone tried welding the cluster shaft to the case. If you want to save the tranny, I think you'll have to take it and have someone braze the crack and likely re-machine the bore for the cluster shaft.
