Need help with idle!

Just got a 74 duster slant 6 225 car. This is the first slant six I've ever messed with, so I'm still on a learning curve. My problem is, after adjusting timing, rebuilding carb, and countless hours of adjusting, it still seems very rough. On first inspection it had 3 bent pushrods. I'm assuming when the previous owner got it running after 9 years of sitting, the lifters had sagged and rods fell out of the cradle. Anyway, that is fixed now. It runs smooth and fires on all 6 like a champ. Even had it up around 75mph this afternoon. The issue now is that when you put it in gear it drags the motor down and kills it. I've got the idle bumped up around 1500 rpm, and it will still die when you put it in gear. At this point I'm completely out of ideas. Any help would be great. As far as I know, it's all factory except the 2bbl carb. I know this car ran great in the past, then sat for 9 years, and now it just been revived. No parts have been changed since it last ran back in 2005.