Idle Rpm High in Park, Low in Drive

And a comment unrelated to your problem-
Those 350 rockers are hurting you, unless they're an aftermarket ratio. Chev 350s run a 1.5:1 ratio, Magnums (and their cams) are designed to use 1.6:1 rockers. Swap to the proper 1.6 ratio and that "pretty mild" cam will wake up a bit.