What’s the Best way I should upgrade my truck
 

Well, If youre gonna go further, go with gauges , EGT, Trans Temp, Fuel Pressure, and Boost gauge. Always a good thing to monitor your mill and know what exactly is going on with it. What is your goal for your truck....street performance, towing, race, efficiency? Once ya start modding, its hard to stop. Also keep in mind, your transmission will quickly become the weak link, Auto or manual.....both will suffer......depending on the mods.

^ every single thing he said. Start with gauges for a baseline, and give us an idea what you're trying to accomplish. Almost every dollar I spent on my truck was with the intention of safe towing, if I'd wanted to win races I'd have gone in a very different direction. Break dynos? different again. Power is cheap at the stock side of things where you are now, but it gets expensive fast once you burn through the free mods and you don't want to do things twice.

Pull the fuel plate out, slide the AFC housing all the way forward, and try to figure out how much more power you want than that while saving for the transmission rebuild you're sure to need momentarily.

(p.s. think in the thou$ands)

You can still have good power, economy, and efficiency without breaking the bank, and staying on the conservative side. I did exactly that with my 06 CTD, great towing, economy, good power..... and I can still light up the duallies and create a hell of a smoke screen!!!! But, I will admit, I had to have the trans rebuilt with stout parts, because it gave up the ghost...…..and my mods are conservative.

The CTD and 12V are apples and oranges, you can literally transform a common rail with no more than a tuner.
All you can transform cheaply on a 12V adding power is transforming the factory slushbox into a boat anchor... unless you fortify it first with at least a billet convertor and some line pressure, which still won't make it as good as the stock trans that came in an '06, and cost about what a tuner does.

JBear, Im in awe of your sig. I'll bet your rig would change the rotation of the earth!

Heh heh... not quite! Aside from being way under-fueled it's set up pretty well, but it's no dyno breaker. Thanks though. It's nice to have a little outside validation for my insanity, I'm a poster child for inability to quit while behind.

I cant help but say Vroom!

and to answer your question it totally depends on your truck and what you actually want to do with it.

first build some kind of transmission. a decent converter is a must along with some kind of valve body modification as previously mentioned. this will hold 400 not abusive HP all day for years.

IMO 450HP on a 12 valve is all more you want to do with a single turbo. the truck will start to have major daily driving short falls. towing compounds like Mr. bear has are wonderful. and I stress wonderful if you want your old stuff to pull like the new stuff.

5x12 injectors get you into the 400's and 14's into the 5's

basic fuel mods, gov springs and afc tuning are essential.

just depends on what you want.