They had a deal on mythbusters about that. With all the test they ran they found that the tailgate down caused more drag then the tailgate up. i'll try to find the episode cause they showed how the wind in the bed moved diffrently.
You need to upgrade your fuel system, get new bigger injectors, most likely a bigger hpop or a dual hpop setup. Then you will need a bigger intercooler, then you have to hope that you have the fordged rods and hope the engine don't go boom.
With the mods you have done the only way to go now is bigger and more oil.
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