Roller LA cam into Magnum?

Absolutely. It's the best way to get a mechanical fuel pump onto a Magnum, once you get an LA timing cover on it.
It's actually a nice mild upgrade for the Magnum, as the LA cam specs are based on a 1.5:1 rocker ratio, and the Magnum uses 1.6:1.
Truth be known, the 318 roller cam is a bit hotter than the 360 cam, and makes a nice street cam for the 5.9, or 5.2 for that matter (okay, the cat's out of the bag, now I won't be able to get any cheap 318 roller cams anymore... :().