ok that certainly helps.
First of all make sure the truck has a fresh set of factory IRIDIUM spark plugs in it. I would also look into a BORLA exhaust system for this one, there is not a direct fit cat back system for this truck so you are going to have to get a custom welded setup for this, removing the factory baffles and resonators frees up a lot of power on these.
realistically if POWER is your main focus you really should have purchased a 2006 or newer GX470, the newer ones have a DUAL vvt-i engine with a lot more power.
A supercharger is the best way to add the most power to this engine, problem is that we have seen issues with the rear main crankshaft oil seals blowing out on these and occasionally the pistons come apart in the engine, because of this toyota/lexus pulled the TRD supercharger off the market for the 2uz engine. You would have to look into an aftermarket one for this reason, I would take a look at this link:

http://www.bulletcars.com/superchargers/toyota/lexus/4.5-litre-6-cylinder-1fzfe-landcruiser-80-series-100-series-4.7-litre-v8-2uzfe-lexus-lx470-supercharger-system.html