99 5.9 should be a VP44 pumped 24 valve Cummins. There are TONS of upgrades you can do. It’ll probably have an HX35 turbo, which can be easily upgraded, larger injectors, performance tune, you name it, it can be done. There’s plenty of folks who crank these motors up. First step would be to read the stickies at the top of this forum:
https://www.cumminsforum.com/forums/...discussion.18/
Then, go read some performance threads there and pick your poison.
As to reliability, you need to know how hard you’re pushing the motor in real time. A set of gauges will be in order in addition to mods. EGT, boost (which you have), temperature (actual numbers, not a H/L), etc. Not sure how much room you have, but fuel and air to match will be a huge jump for you. Larger injectors and turbo or compounds will get you there in a hurry. As long as you aren’t running high egt for extended periods, cranking the timing past a certain point, or overheating, that motor can take a lot. Cylinder pressure is a killer of head gaskets (lots of timing).