Increasing the intake length by an inch will not give any real gain in torque. "Spiral" patterns and stuff really won't do much either, because the airflow will change when hitting the runner, bowl, intake seat and then combustion chamber.
When those things claim to change air pattern in the combustion chamber, they're completely full of shit.
The only thing worthwhile for you is to swap out that 3.9L, or drop the big bucks for forced induction. Those two options would be the biggest bang for the buck.