Too much of both can be a bad thing. If you dump in a bunch fuel in all at once it is going to bog and run like crap. A 600 CFM carb while big for a six can work if you tune it right. I would start scouring web sites or hemmings for a 4 bbl manifold for a straight six. They make them, just have to search.
If you do it right it should wake up the six without too much of a fuel penalty. Definitely an old school approach.
You can get a manifold from offenhouser or clifford performance or I think stovebolt engine co. But if you got the carb cheap I would sell it and buy either a 390 or 500 cfm it will run a lot beter then drowning it with a 600. Good luck either way
I had a 390 Holley/Offy intake on my 230" 6; it wasn't the one most people recommend, and it was way fat just about everywhere. I played with restrictions in the idle cicuit and jetting up and down and got it much better, but it still was unimpressive. You all know how I feel about Edelbrock's so we won't get into that again, but a 600 on an inline is too much in my opinion. For some reason V-6's seem to like larger carbs, but not Chevy inlines without a lot of other modifications. Something like a 32/36 Weber would probably be a better choice.