great job at getting Guinness off tap at a local. The Guinness lie has been going on for too long.
Anyways, it seems as though your only problem for gassing a beer for a nitro tap is Nitrogen gas. Not fittings, etc, you'll need to gas your brown to the same levels with a 70% nitrogen mix, and then make sure the beer is served under it too, or use a "Guinness gas". Another thing, one that gets my goat: make sure the staff serving (if they're young) understand that a nitro beer takes time to pour, and not to rush it.
Brew Before Dishonour!
Five Islands Brewing Company