a 600 CFM Carb would be fine for that set up. in the alternative you could rejet that carb to get it tuned to the engine.
Carbs stated CFM level is what can be drawn thru the carb. It will not flow those numbers on a smaller engine, rather, the air velocity will be slower due to the open bore. (Try and drink your soft drink with a piece of garden hose instead of a straw and you will understand LOL)
The right sized carb will allow better upper rpm performance and better low end torque. too big, but properly jetted will clear up the running rich, but it will not clear up flat spots and stumble completely.
850CFM will supply enough flow for well over 425 horsepower. My rule of thumb is simple 2 CFM per intended horsepower generated. ie.,
a 300 hp engine needs 600 cfm
a 350 hp engine needs about 625-650 CFM
425 gets the 850
and 500+ starts into the 1050cfm
this rule of thumb only works with common engine sizes running up to about 7000 rpm maximum
as power is produced thru extremely high rpm, CFM of flow would have to go up
none of this is applicable outside of NA engines
