Drop in a K&N air filter and keep it clean... You can modify the airbox for a "Do it yourself" cold air intake but not sure what your airbox looks like. Also, as FishHauler stated, add a good exhaust whether you replace the cat and muffler or do a complete exhaust kit... These to things get more air in and more air out which will increase HP maybe 5-10 horses... Also, tune it up, change the fuel filter and run some "Seafoam" through the tank 300 miles befor your next oil change and you will have that truck running awesome!! My buddy has that straight 6 in his work van and it hauls Bass!!!
Good Luck!