My comparison on a lot of this stuff is the Chevy 300HP 350 engine. It had an iron low rise quadrajet intake, stock non HP exhaust manifolds tiny flat tappet cam and 9:1 "or so" compression. Tons of magazines and independents have done lots of dyno tests on those through the years and Chevy's advertised HP was right on the money. Drag strip times even back it up.
The 5.9 has better heads, a roller cam and more compression and longer stroke. I believe with a good intake, a 750 DP and headers, it might make 350 spinning it to 6K. It would certainly be well past the 300 mark.
