Idle Rpm High in Park, Low in Drive

Hook the vacuum advance up to ported vacuum. Think about it. At idle is when your manifold vacuum is at it's highest, and that's when you're pulling full vac. advance now, probably to the point where your mechanical is kicking in. As you increase the throttle, manifold vacuum falls off and decreases your advance. Just the opposite of what you want. Hook it up to ported vac., and you have no advance at idle- vacuum signal increases with rpm and so will your advance- just what you want.
As far as your timing mark goes, are you running a Magnum or '70 or newer timing cover, or a pre-'70 timing cover? Is your water pump a driver's side (early iron pump) inlet, or a passenger side (later aluminum pump) inlet?
If you've got an iron pump on a post '70 timing cover, or an aluminum pump on a pre '70 timing cover, then the pump inlet is covering your timing marks. They're there, just covered up.