I would change thermostat first to eliminate that as a problem. I have read a lot of post with the same problem of hesitation when using the Edelbrock RPM AIR GAP. It is not a good intake to use in cold weather so with the temperature running so cold that is probably the problem. If you can get the temperature up to 190-195 it will probably run better. Experiment with the timing try setting the initial at 16-18 degrees. You will have to limit the mechanical advance to 18-20 degees. What are your camshaft specs? What distributor are you using? Read this Wiki article on setting your HEI and timing. Also there is a section in the article on adjusting the transfer slot on Holley carburetors. Increase the initial timing and close the front throttle plates. http://www.crankshaftcoalition.com/w...EI_distributor
. BTW you will need a timing light with advance.