What I've learned about 5.9 Magnums from the two I built and one helped spec:
Ported heads/cleaned chambers definitely help 3500 RPM and up, help efficiency, and reduce detonation tendencies.
No need for new cams. A regrind works just fine.
One cam 204/210 112°LSA 110° ICL butter smooth best 22mpg in 72 b body @ 2200 RPM over 600 miles trip @ 75mph. Rebuiler short block with stock style Magnum pistons. CR not measured. Runs premium only because he puts ALL the vacuum advance in it as soon as possible. Carbureted with 650 CFM Holley and Performer RPM and cast iron exhaust manifolds.
My son's Dakota R/T club cab 203/208 107° LSA 102° ICL 22mpg @ 75 mph over a 800 mile trip. 13mpg delivering pizza, stupid torque, power DONE @ 4800rpm, you could tell it had a cam for about 10 seconds below 45°F then the computer hid it, bullet spec'd that cam. Stock untouched short block with .028" gaskets. Ran great on 87 octane, no ping ever. Very modified kegger runners set for 5 RPM resonance, shorty headers.
My 85 ram 212/218 112° LSA 108° ICL Lots of positives and lots of negatives. Requires premium due to compression too high for the small cam. Has impressive low end torque due to the cam/compression combination, linier power to 5600 RPM (PCM shutdown RPM), really needs me to install that MS3 setup I've had built for years to have a proper tune. Stock short block with extremely milled heads to make 10.2 compression (43cc chambers). Intake from Dakota R/T, long tube headers.