Did you check on:
You want to be sure the mechanical advance is not coming in when the engine is idling in Park. If it is, the idle speed will drop when the engine slows when you put it in gear- and that slows the idle speed even more.
Check by disconnecting and plugging the vacuum advance, then see if the timing drops when you put it in gear. Use a helper, or set the parking brake and chock all the wheels securely if you try to do this by yourself.
You might need to increase the curb idle speed if the idle drops when you disconnect the vacuum advance. The idea is to simulate the same rpm the engine is at when in Park, then recheck when in Drive, without the vacuum advance being in the picture.