overcarb'ed super six??

A carter BBD flows 280cfm and the 1920 holley 1bbl flows 190cfm.

I would think it is probably jetted to lean, or you have a serious vacuum leak.
I have no experience with street demon carbs, the carter BBD would be a better choice.

As for headers, I ran them for 5 years on my Valiant as a daily driver. If your choke is working on your street demon you will be fine in the winter. I had a 465cfm holley with no choke and it would fire up instantly at 10*, I'd give it a minute to warm up and drive right off.