Based on the 4" diameter and 120 psi the air shocks have a capacity of about 1,500 lbs each if they are vertical, if they are not vertical the capacity is reduced by the angle. This is considerably lower than your front axle weight. You would have to weigh you'd truck to see what the front axle weight is. For a class 8 truck the max front axle weight is 12,000 lbs of the capacity of the tires, which ever is less.
The shocks on a truck are only there to dampen the suspension.
Air shocks and of coil overs could cause you undue scrutiny if the truck is inspected or a DOT inspector happens to see the truck. Worst case scenario would be an accident and the subsequent investigation.
You should do this correctly. Due to safety issues the front suspension on a truck is not adjustable. If the front suspension is sagging you gave probably overloaded the springs. This can have some very bad consequences with the drive line geometry.
You need to get the frame so the geometry is the same as the stock truck. This can be accomplished by rearching the springs, adding blocks or converting to full air ride.
The blocks on the front axle if a class 8 truck are actually short very thick leafs added to the spring pack.