I currently have a 600 cfm holley on my motor, and was wondering if a 650 would make a difference? Have a 350 motor, barely above stock range.

 

Not much.

 

I doubt if you could even tell the difference. I'd also question going any bigger with a fairly stock engine. You might lose whatever bottom end power you have now, even if you gained a little in higher RPM's where you'd rarely use it.