With the car NOT running look down in the carb and see if the rear plates open all the way as you open the throttle wide open, sometimes they can be catching on the gasket or manifold.
There is, I believe an adjustable plate you can buy to put in your dizzy to adjust the mechanical advance.
Look here:
http://www.4secondsflat.com/
They also offer a service to re-curve your distributor.