Tripple carbs are a very tricky thing if not tuned correectly. I havn't done it myself but am in the process of running triple 40mm Solex Carbs with a Mikuni Manifold. However, if you havn't had them rebuilt in a while, it is highly suggested that you do so, I know a guy on Ebay that sells rebuild kits for the carbs at a good price.
Nissan Motorsports (The American One) should have the Mikuni Service Manuals. There is a guy named Gerald at Glendale Nissan in California and his phone number is (818) 547-5550 at the dealership. Ask for him andw then ask for a Mikuni Service manual if they have it.
Nissan Part Numbers that may be of some help to you...
Mikuni Carb Rebuild Kit 99996-S1044
Okay, I don't remember the one for the rebuild manual, but go ahead and ask.
I can't seem to find anyone who can tune Tripple Carbs at an affordable price around here.