The 350 in my 67 starts hard, usually sits 2 wks or so. When it finally starts it runs great in park, but as soon as I pull into R or D it wants to die. I have even turned the idle screw way up. I have recently rebuilt the carb as it was gummed up from sitting a couple years while doing body work. I also think my vacuum advance is not working. The trans is a powerglide. Hope I have enough info included for you to help. Thanks,Steve

If your vacuum advance isn't working than it's not helping but, I would check for a vacuum leak. My 66 ran perfect in park or neutral but when you put it in gear it would immediately die. The trouble was the vacuum line going to the transmission.

OK, I will check that out right now, thanks. P.S. How can you check vacuum advance?