Carb for 340

Let me start off by saying I have looked through a few different articles on this topic and have read though them quite a bit. This is to more so get a second opinion on my specific set up and what you guys would recommend. I'm not just being lazy and not searching the forms. I love coming here because I always get really quick solid answers. Thanks guys :D

Now on to my question. I have a 1970 340 that seems fairly built. Everything was done by a buddy of the previous owner so I don't really have any specifics on the cam that was put in it. It does seem like a very chunky can. It has a very rough idle to it. (the ignition timing is set correctly as well) it has long tube headers with a full 3" exhaust. It has a Weiand intake manifold and 360 J heads. I looked around at a lot of the carbs people were recommending and I really like the Edelbrock Performer 750. However 750 CFM seems a bit much for a 340 SB. I don't know if the level of mods on this engine would justify a 750 CFM. All the CFM calculators I've seen have suggested 600-650 CFM, however there's no way to take any of the modifications into play in those either. Any help would be greatly appreciated. Thank you