5.9L Magnum build with questions

If you keep the lift on the low side, Hughes Engines has a trick retainer and spring set for the Magnum heads that allow a .540 lift. How long the valves hang open is more of a concern. A long duration cam is part of that issue.

On the cylinder heads, there are two heads available. The iron EQ or the other make which the name escapes me at the moment. But, again, Hughes engine s has these iron heads. OR the new Trick flow heads. Way pricey by compare BUT a massive ability to take you much further down the performance line since it’s performance ceiling is way way higher.

Besides the TF heads cost, they are set up for both engine series. There is one caveat to this. The rockers used are, of course, pricey and designed to ether purchase for the Magnum or LA style.

You didn’t t mention gear ratio or the cars weight and tire size. So a cam is hard to recommend. Another thing about a cam and how well it operates and how long in RPM terms it delivers power would be the cylinder heads. Stock port limit RPM potential and well ported aftermarket heads allow more than advertised RPM.

Also, is this car to be a daily or often driven? In what conditions?

Once you make a head choice, a cam choice gets easier.
I wouldn’t rebuild the OE heads. I would just use them as is, stock, with a Hughes Engines trick spring kit and a mild cam. Or move right to the trick flow heads and never look back.