Does anyone know how i would be able to put out 400hp from the standard 4.0L V6 Tacoma engine?

400 hp would be a lot to ask of that motor. While you might be able to get that much out of it (anything is possible with enough boost), I dont think it would have any reliability.

What do you think would be the most hp I would be able to get out of that motor, while making sure not to blow the motor out? 330-350?

On the stock connecting rods and crank, Id say probably 350 would be about the limit.
If you rebuild the internals of the engine with stronger parts and turbocharged it, I think you MIGHT be able to get 400 out of it.
Like I said, anything is possible with enough boost.

So if I put in forged pistons and rods in they would be able to handle it. What other parts would you recommend if i was building from the inside out. Stronger cam shaft? Stronger Headers? What else would you recommend?

Basically everything stronger. You would want different cams that are made for racing and high-compression heads.
Basically, call TRD on the phone and tell them you want the most badass 4.0L their sick minds can come up with.

Yep good call. A friend of mine did it. Called TRD and went for the gusto. I never did hear what he ended up with in the way of HP or RWHP. I do know though that after he built the motor, and a transmission to handle it, he was in right to the tune of $14,000.
Good luck to you. (IMG:style_emoticons/default/w00t.gif)
~Mike~