Idle Rpm High in Park, Low in Drive

In answer to your original question.

It is normal for a carborator car to drop rpm when in gear.

Go to mymopar.com and down load your cars Factory Service Manual. It's free no excuse.

Also
Look at the (very corny but informative)

Chrysler Master Tech Service Conference Library – MyMopar

And watch the engine tuning movies. You would be surprised at how similar these old carborated cars are to modern EFI electronically controlled cars. They are just mechanical computers