A few months ago I bought a 2008 Silverado LT 2WD ext.cab std.bed with 5.3L, my plans for the truck is a mild performance truck with an aggressive off-road look so I do plan on doing a leveling kit and slightly bigger A/T tires,
Truck is rated at 315hp factory, my goals are, (I don't plan for all 3, just which ever one comes first) 400HP, 5.5sec 0-60, OR 13-14sec in the quarter, so far I've only got a Hypertech MaxEnergy tuner, and just got a Powerstop slot drilled rotor performance brake kit, I plan to install tomorrow.

My plans are cold air intake, aftermarket MAF, TB spacer, larger TB, true ram-air hood, MSD ignition coils, hi-flow cats, cat-back exhaust, once I do a few upgrades I plan on doing a custom tune, and two maybe's are headers and 3.73 gears (3.42 factory)

--So questions are best guess where I would be at with those upgrades? What would it take to get roughly 5.5sec 0-60? And can my factory transmission handle 400HP

Thanks for the help

Those aren't upgrades. Those are you pissing your money down the toilet and in some cases downgrades.

The only real mods worth a damn n/a off the top of my head would be live custom tune (not mailorder), headers, cam, gears, aftermarket torque converter.

To achieve around 400 (crank hp) in a 5.3, you will need a small cam, headers, Airaid tube and a good tune. Which will equal out to around 300-320 rwhp depending on cam. You will need major valvetrain, headwork, large cam to achieve maybe 380-400 whp, which your looking at $6-8k and wouldn't be street friendly. If you want over 350 rwhp its cheaper to do a 6.0, or forced induction. Also your 4l60 wont take much over stock ratings. A mild built one should handle 400 crank hp. And i agree with GMCtrk, those are a waste.....coils, MAF sensor, and TB spacer are worthless. Stock is just fine. Check out my Sig, I should be around 320 RWHP when i do the cam swap.

i made about 390 before i went turbo. I had port and polished 243s, 226/230 cam, 3600 stall, headers and air intake.

Listen to this guy.

Alright thanks for the info, gives me some things to consider, I thought about a mild cam, but I don't really wanna go internal. How much of difference do u think 3.42 to 3.73gears would make, I've thought about 4.10 but I don't wanna kill my fuel economy? My old truck I went from 2.73 to 3.42 and made a huge difference in acceleration but little to no difference in fuel economy, but that truck had 33's so it probably needed at least 3.42's

If you want a cam that wont lose any lowend stay under 210-212 intake duration and 218-220 exhaust duration. Max lift for stock heads is around .551 and 112-116 LSA depending and how strict your smog sniffer is and how you want your idle and powerband.

You will have to tear the engine down to a shortblock to do this. You will need to change lifters, lifter guides, valley cover, valve springs, pushrods, new head bolts, 3 bolt 4x cam gear, and i recommend LS2 timing chain and tensioner.

Gear size depends on tire size. I run 31.6" and run 4.30s and didnt lose any mpg. I will end up going to a 32.8" tall tire. But in my opinion 4.30s are ideal for towing and highway for a 4 speed with my tire size.
My opinion
Under 31" - 3.73
31-33" - 4.11 or 4.30
Over 33" - 4.56
This is if you daily and drive highway

LT's, torque convertor, a real tune, servo's, gears would get you a lot faster in the 1/4. Off road tires would definitely not help of course.

to use a golf analogy you are missing the mark by a couple degrees and it's causing you to slice way off course. First things you will want to do are the supporting mods.
1) ditch the handheld tuner and get a custom tune from a reputable source. Tell us where you live and we can give you some good options.
2) I am guessing you have the 4speed automatic(4L60e) you will want a larger trans cooler and the Vette servo ASAP.
3) gears are a great idea. This will give you huge bang for your buck. I think guys have given good opinions on ratio to tire size. I personally love my 3.90 gears, but would recommend 4.10 or higher if you go with larger tires.
4) if your doing gears get a posi/locker done at the same time the factory one is garbage
5) boltons. Longtube headers, magnaflow muffler, cold air intake. These will help get you moving. I wouldn't do a complete catback as its a waste of money when stock piping is 2 7/8" and a 3" exhaust will I've you very little bang for your buck.
6) the next mod is a torque converter and cam. I put this together, because they should be speced at the same time. This is all about how much effort and how aggressive you want to go. For a mild setup the tried and true 212/218 low lift is right up your alley. Matched with a circle d 278 mm budget stall you will make great power for reasonable money.
7) if you need more come back and see us, FI, internal work, more cubes etc is a whole different can of worms.

easiest thing would be to do long tubes back, mild cam, and custom tune.

i would look into the BTR truck stage 3 cam along with springs and afm delete.

if you really want to have fun get those 799 heads ported by some one good and will defiantly make it.