Just to add my 1-1/2 sense here (not cents), On a box stock cj type Cleveland, particularly if you are running the low compression version, you will probably be happiest with a 600 or 650 vacum secondary carb.
HOWEVER, I want to remind the old timers here and inform the newbies that the Ford Muscle Parts program highly recommended an 850 Holley double pumper for the 1970-1 351C 4v. Of course this was for the high compression Clevelands since this was 1971 and the compression ratios weren't dropped down to 8.0:1 yet.
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