@Shadyacresmack Is it the truck in the pic? It has 460 on the door. Engine or door swap, or did you mean 460? 460 to 525 with ECM tuning is possible, maybe. 400 to 525 with no hardware upgrades isn't.
Is this old mate who's told you he can tune it 120hp+ with no hardware upgrades?
Personally, all I can see happening from a copy and paste tune, is that it'll burn up your engine, and your wallet. If he said " bring your truck in and we'll tune it on the dyno" then maybe you'll have half a chance at a decent result.
Can't talk from experience, but by all accounts, Rochester or Antrim and maybe others, could get you sorted for injectors and turbo and make some good gains without loosing too much reliability. As Joey said, 400 to 460, pretty sure you'll be pulling the cam to swap keys, plus turbo and injectors. That is if you have a 400.
I'd start at the basics, check for boost leaks, fuel pressure, valve clearance. Get it plugged in and verify the engine settings too. Have you got a boost pressure & pyro gauge in the truck, these readings can tell you a lot about what's going on if your down on power. I'd say their essential if your running in hilly country.
As mark said, diff ratio could be crippling you too, at 90k I wouldn't go taller than 4.3 diffs with a .73od box. I've got 4.56 in a 155k rated CLS 460 and even at 90k its not breaking any records. But it'll do it all day long regardless.
Two things to remember, there ain't no replacement for displacement. And there ain't no free rides, you've gotta pay for the ponies.
Adam