IMO I think that you need to tell us what you have in mind for a cam and what amount of head work you plan on having done and is the compression ratio staying the same or is it going to be increased? I don't ever run anything smaller than a 750 VS carb on the street, and alot larger than that on the track. In order for anyone to tell you exactly what carb you really need is a shot in the dark until we know a few things as I mentioned above, as they all have effects on the carb. A 360 that I did with mild head work and a XE284 cam and a Torker II intake and shorty headders needed a minimum of 750 cfm's and we ended up using a 830 cfm carb. Throttle response was great and crisp as you would ever need one. We tried smaller carbs and they were flat just above 5,000 rpms, because they just couldn't flow enough air to satisify the engines needs.
If this is going to be a street only engine and less than 5,000 RPM's 98% of the time then I might consider a 650, but don't expect any big power from the engine. And as I stated before this would be in stock form only, without any considerations for improvements. But from what you stated that you have intentions of modifing your engine then I wouldn't even consider anything smaller than a 750 VS. In fact Holley recommends this for there street avenger carbs,
570 cfm's for 300 HP
670 cfm's for 300-400 HP
770 cfm's for 400 + HP
870 cfm's for 500 + HP
you can get anyone of these with electric choke. A buddy of mine had a 670 on a stock 454 and had problems pulling hills with a trailer behind him. This is in a Chevy Dually, he stated that he was getting 10 MPG with it but couldn't keep up with traffic on the interstate when a hill came up. Then I loaned him my race 780 cfm carb VS and he said man what a difference. Sure his gas mileage dropped but it was because of his right foot. He told me that he no longer had to keep his foot on the floor to pull the trailer, and that when pulling the trailer that he could pull hills at half throttle. He said that he could do 70 mph and it not back off at hills. And the engine was only turning just over 3,000 rpm's.
So my point being that reguardless of engine size a VS 750 is one of the best all around carbs for the street and power and IMO I would use a 770 street avenger. You have to look at it this way also, that your only running the engine on the primary front 2 BBL's most of the time, and if the engine needs more then this can be adjusted by the secondary diaphram springs for when you want it to come in. But it's a simple change.
On my 322 engine with 302 heads I don't run anything smaller than a 750 and in the last few weeks I have had a 9375 1050 Dominator on it, that a friend of mine was having problems with on his 540 BBC engine. I cured the problems with it and he told me to run it so I did. The result was that without dejetting it that it ran just as fast as the 750 did but believe me there was alot left in the engine with this carb on it. Had I been able to tune it to the engine it was on. I would say that with this carb on the little 318 .030 over that it would have run 1/8 mile times equal to low low 11's on 87 octane gas in a 3,330 lbs duster.
I guess I look at carbs differently than most do, I don't like to put the minimum cfm carb on a engine but rather I allow a couple of hundred cfm's more. And this usually is seen in the time slips at the track. If more airflow didn't or doesn't make more power then why did they put Q-Jet carbs on the 231 V-6 engine that Buick ran in there Grand National Cars. I had one of these cars without the turbo and it ran very well, and in stock trim could smoke the tires. The main thing here is to use what ever carb that you want, it's your car/engine and you have to live with what ever you choose. But I know what I would do. And this is comming from someone that has built many different engines and for different purposes and what they required to make them work well.
Oh ok 1 more, my 318 that I had in a 4,000 lbs truck was getting 17 mpg without a OD and it was a 750 VS holley. This was a stock 318 engine and a 727 transmission. And it passed 04' emissions better than the new cars did. It was a 3310-1 and had 78 and 84 jets in it with a 6.5 PV and the air bleeds were 72 and 32 idle and power, with the idle mixture screws turned out 1 turn. The engine had 20" of vacuume.
