There are alot of things you can do to it. TheFermanator is on track with it. The rest of the douchbag's that say "just put a cummins in it or don't do anything", should stop muckin' up your thread. The same clowns that say that probably also argue STILL that the 6.2/6.5 is a converted gas engine, when if you have ever had one apart you would clearly see that it's not. Not to mention a simple amount of re-search would put that wives tail to rest.
"I got me a 12 valve with 370 marine's and an HX40, yee haw it's the fastest damn thing around!! I know so because it smokes like a train and that's super cool." yuk yuk, please
Check your balancer for slippage, they are usually in need of replacement.
Find some Bosch Marine tips, or install new tips and lower the popoff pressure to 2000. Advance the timing, gauges, headers to a better turbo. Don't let fuel pressure drop below 12 lbs, put a lift pump on it if you need to. Don't take it up past 1200* for more than a couple seconds. Especially since you don't have any oil squirters.
Do you have stock pump on it? DB2829? You might have got lucky and got the DB4911. If you can fabricate headers, that would help immensly. If you want to spend some real cash, get a newstyle block with oil squirters, install a girdle, then you can make some real decent power. Just depends how much you have to spend. They are GREAT engines, you just have to be realistic with their limits. They aren't a powerstroke/duramax/cummins. But they can make good power and are cheap to fix/maintain.
Do you know if the engine has had head gaskets or any internal work done?
Sorry for the rant, dude. I just get so sick and tired of people saying "12valve or nothin'". I have two of them, great motors. Everything has it's positives and negatives.
Mike