I know H/C/I all at once may be more effecient, but money doesn't grow on trees. First of all, how is it running, I mean, has it had a full tune up. Fuel filter, fuel service, cap, rotor, wires, spark plugs, induction serviced, radiator flushed, trans fluid clean, timing set, ect. This is one of the biggest mistakes people make. They spend all this money on mods with a engine well overdue for a tune up, and wonder why they loose to a car with less mods.
With 4.10's your engine is revving fast alot. Something spinning at 6000rpm is spinning really fast, and do you know how much energy it takes to spin something that fast in a period of 3 or less seconds from idle? Especaily since some accesories are overdriven? Well, underdrives would definatly let more of that energy go to the wheels.
An large intake on stock heads you will loose power. Probally a ported lower will do fine for a while. The upper flows near 200cfm stock, but the lower only flows about 130cfm. TMOSS does a good job from what I hear. Then you'll need heads, but that's another project and a couple more paychecks.