Hi there. Hellwig makes an "add-on-leaf" to help with the overload issue. It and the airbags really only add to bed height and load stability. They do not add weight capacity.
The basic axle, transmission, brakes etc are what really determines weight capacity.
To pull the amount of weight you are talking about the weight distribution is a must if you haul any distance or on public roads.
As far as air-bag capacity, there are more than one model made for the Titan. Mine are made by Firestone "Ride-Rite" and have a maximum 5000lb capacity. Google up Firestone air-bags and shop around. Same for the Hellwig helper spring.
EDIT: UPDATE FOR URLS
Nissan Titan Firestone Air Bags 2004-2008 - Suspension Air Bag Helper Springs Hellwig Products | lp-15 | Sway Bars, Air Springs, and Leaf Springs
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