I think it should idle better than that in 20 - 30 degree weather. Sub-zero: yeah, that's a challenge, but you can set it up to run okay in those cold temps.
First thing's first: confident that your warm idle mixture adjustments are right? Base timing is right? What RPM does it idle at cold and full choke (i.e. when you first start it in the morning)?
Before you start the motor, push the accelerator to set the choke, then pop the hood and look at the right side (driver's side) of the carb and note the position of the fast idle screw on the fast idle cam. At least close to the middle of the two lines?
Then go to the Edelbrock (actually this is a link from Summit) site and get their choke adjustment instructions (very bottom of this PDF).
http://static.summitracing.com/globa...-1478_inst.pdf
I think folks don't always understand how to adjust these things but really, if it's not all totally worn out, it's a decent setup.