the threads in the fitting would not add strength to the connection. It does provide a tight fit for the bolt which prevents movement, and movement is what usually starts the cascade of failure. If it were mine, I would have the fitting drilled for a pretty tight fit (normally clearance holes leave a bit of slop) that would provide a good guide for tapping the mast. When you install it, coat bolts with red Loctite or the equivalent. This helps prevent loosening and corrosion (it seals the threads). Red Loctite is considered "permanent" but you can always release Loctite by heating it.

However, as always, do what YOU think is right. You are the one looking at the actual parts and dealing with it.