It depends on several factors. I've started with a C code also and made some changes which helped but admittedly need more. I went to the Performer intake manifold, Holley avenger 570CFM, Performer + camshaft, roller rockers, dual exhaust, pertronix ignition and my performance is better.
In your case, how many miles on the motor, are the heads stock 66 heads, dual exhaust or single exhaust? Are you using the stock log exhaust manifolds? If the motor is high mileage, heads stock and using the stock log exhaust manifolds, then in my opinion your carb will be on the big size if going to 650CFM. You'll be better off with no more than a 570CFM. If I had it to do over again, I wouldn't buy a Holley, and probably would go with Edelbrock, because I'm not happy with my Holley. Edelbrock has kept up with technology more than Holley and my personal opinion is Edelbrock is a little better, especially since you mentioned going with the Edelbrock manifold. Edelbrock went to great lengths to build and test these components together to get the most out of them. The Performer is a good street manifold and if you were to go a step further and replace your cam with the Performer + this would also boost performance. If you have a high mileage motor, think long term........so you don't repeat your steps, which will spend more money. I'm speaking from experience, as this is what I'm doing right now. I started out with the C code, rebuilt it and left the dished pistons, mistake, should have changed out to flat tops. I rebuilt the stock heads, mistake, should have at least installed larger valves and ported and polished those heads. I kept the stock log exhaust manifolds, mistake, at least should have gone to HI PO manifolds. So you can see by my mistakes I'm trying to get you to set a long term goal on your motor to save time and money. Bottom line, if you have a worn out stock motor and over carb it, you performance could get worse.
65 Coupe A Code, CSRP front brakes
C4 with shift kit
289 4V Autolite 4100 1.08
Edelbrock Performer Intake
Edelbrock Performer + Camshaft
Tri Y Headers
Magnum Roller Rockers
Pertronix Ignition, Electric Fan