well, i have been doing the exact same thing with my 65 stang with 302.
when i bought it , it had a 750 holley 4160 on it.....ran great but i suspected it of being to rich ( 72 jets ), anyway that holley mysteriously started idling weird......so one day im gonna take it apart.again to see what i missed. then i had a 750 edel on it.wanted that carb for my f-100....so im playing with a holley 600 now. it had 41 jets, way to lean and it ran badly..so tonite i went and got some # 69 jets, and maybe in a week or so ill see how it runs...........
about the rich running of yours.......another thing thats a must on holleys is the right number power valve......most holley power valves that come in the carbs open at between 7 and 9 i believe........if you have a lopey cam and low manifold vacuum at idle, the power valve will open causing a rich condition so check that as well.........put a vacuum guage to a manifold vacuum source and see what it reads.....then buy a power valve that opens a point or two below that ( if you had 8 inches of vacuum you want a 6.5 or 7 power valve) . if you pull more than 10 inches or so of vacuum it shouldnt be a problem.......but check it while you have it apart..........
a 600 cfm should be fine.........even a 750 would work if you tuned it right.so you should be ok......the above formula that was stated above is a good guide, although for some reason it always seems a little small of a number when i use it but i think this is due to the fact that it doesnt take into account thats if the carburator is 100 percent efficient.and id immagine there's enough variables to justify larger carbs than the formula suggests........but it is a good guide and 600's will work on about anything. especially small blocks.