few magnum questions before swap.

Are you using the factory injection or a carb? The Magnum Alt is managed by the PCM although looking at wire diagram it seems like you could hook it up without it. Never tried because everything we build has injection and PCM.

You can use a roller up to about .470" lift without breaking springs assuming they are in good shape. The 5.9 heads have tiny intake valves, smaller than the 5.2 heads. If you can change the heads to EQ or at least 5.2's I would recommend it. We have a .450" lift cam for $199 thats a huge improvement over the pathetic .400" stock 5.9 cam. FYI stock 5.2 cam is .439" lift.
BTW, most 5.9 heads are cracked, we just assume they all are and dont use them.

Bare minimum we recommend is 5.2 heads and cam but always recommend EQ and our stage I cam. Anything larger than that cam needs springs but our stage II springs are good to well over .550" lift. The stage I cam is also as large as you can run without flashing the PCM.