Six pack help!!

Just wondering if anyone can help me out
I have a 68 dart with a good running 340
With auto trans I changed it out to a six pack setup and just can't get it to run good! The carbs are rebuilt
I can only get it to idle in park at around 1400 rpm and sometimes if I give it gas it speeds up but on the way down it stalls? The other problem is if I put the trans into Drive it stalls right out ?? Could it be a vacuum problem ???