I've seen lots of stock 351c's with 750 carbs and they have worked fine.
It is a vac secondary, so even if the throttle is stood on the engine is only going to see half the carb until the airflow goes up.
1/2 of 750 is only 375 cfm. That should not cause the off idle bog that has been described.
Though if i was going out to buy a carb for a stock motor i agree a 600 is what i'd go for.
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