Edelbrock Carb Choice

Niether ? Holley. But depending on weather I would just get a manual if you don't plan to drive it when it's real cold outside

I have the electric choke and like it, but I'm not sure it makes much difference as I haven't driven the mustang during cold temperatures. Anyway my vote would be for electric.
Why 600 cfm? 500 cfm is enough until you get a somewhat serious cam, head work, and headers and free flow exhaust. You have not mentioned other engine mods.
Here is a very good article about treating an engine build up as a package. Note that the Ford advertised hp was "optimistic" and the way hp was measured in the mid 60's is different than current measurement methods. The A code 289 used in the mag article was advertised as 225 hp but only measured 128 at the wheels.

http://www.mustangmonthly.com/howto/26821/