I wouldn't jump to cam swap right away.
The stock truck motors don't like to rev because of the intake manifold design. "beer barrel / Kegger" style intake with LONG runners for torque, and a marginal sized throttle body. These, coupled with a mild cam, create an engine that is built for TQ and low RPM
Once Intake and throttle body are addressed, the engine will run into the upper RPM range much better.
Likely you will use a standard carb application intake manifold, and 4 bbl carb. These 2 alone will make a big difference.
Cam swaps on Magnums are kind of pricey. The cam is 2-3x what you would pay for a basic flat tappet hydraulic cam and lifter kit. ALL of Comp cams offerings require new valve springs due to coil bind of stock valve springs, and Comp does not offer the springs, you have to go to Mopar Performance (unless you can find springs based on open and closed seat pressure, and ID and OD specifications) I would not be too surprised to see 400-500 bucks wrapped up into a Cam/Valve springs combo on Magnum motor.