Missing under load

Some of the problems the magnums have are,
Sucked in plate gaskets on the bottom of the intake causing a lean condition, test for vacuum in crank case with everything sealed and vacuum gauge on dipstick tube , should have pressure no vacuum .

Try disconnecting egr hose , they can pull open and cause problems

You could also be having a problem with the fuel pump module , they can go bad and register fine on a pressure test and suck air when the gas level is low and put air into the fuel rail and with it being a single line fuel system ,usually the air will make its way into the highest point of the fuel rail which is cylinder 1 and will show up as a misfire on that cylinder.

Weak coil , try a known good one.

Common for screens on injectors to plug up with debris ,rust etc. Inspect injector screen. Or plugged injectors, with a scan tool watch short fuel trims and oxygen sensors going lean during misfire.