Now to be a wise guy ... but there is a threaded hole in the rear of the carb ... did you plug it? Makes for a big vacuum leak if you did not. I had a friend who installed a new Edelbrock 600 cfm on his 350 and forget to plug that hole. It barely ran ... and flooded the car. Check the plug in the rear.
This photo ( off the internet ) had a brass fitting in the place I am talking about.