Fast is all a relative term. You could put in a fully forged built high c/r 6.0 with a screaming cam, but then it won't feel like much after you realize who is faster (lighter) and you'll want more and more and more. Not to say the addiction isn't fun, but it takes a big wallet and a lot of motivation into something that will have little investment turnaround.
If you're really dedicated to your truck though just pick up a junkyard LQ9 and just build it on a stand then swap it in as your iron 6.0 accumulates miles. Or hang a procharger off the front and plow some banks!!
This.
Since you said you plan on having the truck for 5 or so years, you should keep your eye out for a well priced, decent mile LQ9. Do heads, cam, LS2 timing chain, ported oil pump, and NNBS intake mani on a engine stand. Then just drop into place (not hard). Keep an eye out, and dont rush to find a motor. I bet you could score one for $700-$1000. If you can find one bare bones, for cheaper, your ahead of the game, and can build up from there.
Motor work is expensive, but if you can do the work yourself, find as much stuff cheaply/used but in good shape. It's really not terrible.
Hell, your truck, with LT's, ORY, tune, E-fans & 4.56's will feel alot better than it does now.
I am also on the fence, just like to wether to dig into motor or not. I already have an LQ4 with a tune, but I'm undecided if I want to do E-fans, cam(& supporting mods), heads, NNBS intake manifold, LT's, ORY, E-cutout, gears, maybe light weight pulleys, and a good stall, with built tranny. Go big.
OR just do what kyle kind of said, keep motor sort of stock; LT's, ORY, E-cutout, E-fans, gears.
