The big limitations on the 5.9 are
1) intake manifold, it doesn’t tune and cuts HP.
2) intake valve size -add 2.02”
3) compression ratio- a little bit more will help
These three together with the ‘98 Jeep 5.9 calibration work well together.
The stock exhaust systems are extremely restrictive too.
Sadly the LA rocker shaft was replaced for the Magnum engines. I think getting some of the valve train stiffness back would extend RPM range but not sure exactly how much this would help.
