Idle Rpm High in Park, Low in Drive

My guess is that it has a stock mid 70s distributor with a lot of mechanical advance and a similar vacuum can .
So when you set the intial timing via a vacuum gauge you are getting too much timing too quickly . The hotter cam likes more intial timing than the stock one for that distributor but then total timing ends up around 40*+.
This is why Mattax says to use manifold vacuum ... but you need to set intial back around 5-8*. The vacuum advance will bump the idle timing up into the range that the new cam likes and wont over advance it mexhanically .
I bet there is a 15L or 17L advance plate in that distributor....
The right way to do this is to recurve your distributor using a FBO plate and lighter springs. Set the initial around 14-16 and total around 32* . That means you will use the #8 slot on the plate ( it has several different length sets of slots on it ) .
Then you can readjust your carb for idle mixture.. which may be wrong now due to the timing situation .
And take the carb off , flip it over and set the throttle/transfer slot to just about square .

How to limit mechanical advance in a mopar distributor, tuning for street, strip or all out racing, cure that rich stinky idle, win races