I have an engine question for you that relates to your issue.
What cam do you have in there? (Specs?)
Are your heads ported? Headers?

The smaller 600 will increase throttle response a lot. The loss of HP will be on the top end. How much time do you spend there? If it is not a lot, then the smaller carb can be done. I would however choose the Thunder 650 AVS hybrid since the secondary air door is tuneable and will avoid the bog that happens when the AFB suddenly has a W.O.T. application.

Otherwise, the Holley will do just fine. And I would also repeat the use of a double pump for sure.