Idle issues when in gear

Love your wagon ! My 76 Aspen with a slant does the same thing and it also diesels when I shut it off in park but not in R .I have set the timing to an advanced position to raise the idle so I can back off on the throttle screw. It made a huge difference and stopped it from running on when shut off in P. Still runs kinda rough when idling in gear but running is smooth as silk. I'm still chasing my tail over this problem .