Carb problems

a thicker base gasket under the carb will change choke setting which can be corrected by altering the bends in the rod up from the heat riser accordingly. the angle of the throttle return spring will change a bit also. sounds like you have a problem in that part all ready. if you don't have a double return spring... get one.
64 may not have the best set up in throttle pressure linkage either. I'm not sure what year those changed.