Family of 6 future tow vehicle.

A $500 "cold air" intake system will likely get you 0, but maybe 1 or 2 additional hp & tq, at the cost of more dust into the engine. A tuner might gain you 15 or 20 hp at some RPM, but unlikely at peak RPM, and you'll likely then need premium gas. No offence, but you're dreaming if you think you're going to get 500 hp out of that combo.

That said, a diesel in whatever flavor of HD truck you choose will pull better than almost any gas engine in the same truck, and better than your 8.1L. They have more than double the torque your big block does.