I don't like twin blades. Not only as a employee of Hustler but as a mechanic. I know some large firms use them across the country but they have a downside. When you engage your blades there is a lot more weight for your clutch to get moving, plus when you shut down the blades there is a lot more weight for that little brake to stop. Most of the time when you change things around you rob Peter to pay Paul. You need to weight the cost of the modifications.
I don't know the size of the bolt but a good hardware store should have what you seek.