The stock gears and stock low stall torque converter is not going to be very friendly to getting crisp throttle response with a double pumper carb. Just too much throttle, too fast.
Recurving the distributor advance curve will help. More initial at idle always helps throttle responce.
16-18 base inital. 34-36 at max advance. a 18deg advance curve for a mild cam.
the carb will need extensive retuneing of the accelerator pump systems. ( shooters/pump cam)
The temprature of the intake manifold plenum definatly is a factor, especially in the winter.
Cold manifold on a cold day ='s trouble with throttle respose from a very low rpm, especially compounded
by a stock low stall converter and stock sluggish gears.
A afr fuel ratio meter/gauge is great for getting the carb dialed in.
They are never right,, out of the box. A the cold manifold is going to compound that.
The performer rpm is a great manifold but the AIR GAP non heated design is not the best for WINTER driving.
The RPM manifold with the full divided plenum and under plenum heat path is a lot more winter friendly.
The Mid rise style "EPS" Performer is not bad too.
The cold plenum air gap style manifold may have been a bad choice for this near stock motor and winter driving.
Again, the stock gears and stock low stall is not helping.