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One easy solution is to take your truck to a local heavy duty spring shop that can add some bend on the leafs of one side. Tell them to level it up. They have a machine that takes care of the rest. Basically they remove one side and custom add + .5, or 1, etc whatever is needed. Cost ~ $200 to $250. Done in an afternoon. The Nationals are REALLY elastic because there are so many thin leafs in the stack, so it will take them a few hours to get the job done right. But it will look good and does not affect performance in any significant way.
Be sure to fill gas tank to the desired level (half?) and add driver weight (sand bags?) if you drive solo and want perfection. And be sure bed is otherwise empty and undercarriage is clean and free of any weighty mud. Drive easy getting there and don't make any hard turns that might affect the ride height. You don't want anything to skew your results.