Ok, back to the beginning. If the machine was born with a 5.9, then a 5.9 it is. I'm not gonna argue something i cant prove. Bottom line, if you're hitting peak RPM and not producing a ton of blowby, you have 2 limiting factors on the hydraulic side. Torque is relevant to pressure and horsepower is equivalent to flow. If you don't know, have never done, or have no experience with these adjustments, I'd contract it out to a trusted professional. It's very easy to get deep in the weeds with an incorrect adjustment.
If we focus on the engine aspect of it. Disconnect all throttle linkage. Once the engine is up to temp, go to full throttle and hold with a bungee or zip-tie, even mechanics wire if it's easier. Then operate your machine, gauge pressures, write them down. Measure cycle times, again, write them down. Make note if engine produces black smoke or grey and under what circumstances. At what point does engine rpm drop?
Speaking of overlooked engine concerns. CAC piping and joints. All good? Any ruptured hoses or clamps. CAC, itself, good order? No cracks, no oily spots collecting dirt? Fuel hoses? All routed smoothly, no sharp bends, kinks or abrasions? Nobody threw a shovel where it doesn't belong and damage anything? Something a little less obvious, flywheel mass. Did you swap the flywheel/ flexplate over from the original? If it had a Lovejoy, was it in good order? If you lose reciprocating mass, you loose torque.
