Because you don't have enough initial timing you likely have opened the throttle blades enough to uncover the timed port and that's why you are getting a timing change with the advance hooked up.
The 15-20+ initial timing recommended previously will allow you to turn down the idle. But you will also need to limit the total mechanical at the top end by either using an adjustable distributor like the Mopar Performance unit or an aftermarket one or weld up the slots in the stocker.
First step in tuning a motor is to get the timing dialed in before touching the carb.
Also, the 74 318 is a low compression motor and even with shaving the heads you still may not have enough static compression to run vacuum accessories with that cam.