Too Much Carb?

Definitely too big. You are correct in surmising that a too-large carb can be made to run by jetting the fuel way down, but you will suffer in the torque, driveability and gas mileage departments due to the low vacuum metering signal. The Six pack setup used vacuum operated outboards which only opened on demand by a high vacuum signal from the plenum. Also, these carbs were rated differently as previously mentioned and actually flow only 250cfm. The Carter ThermoQuad, while rated at 800cfm, had an airflow operated air valve on the secondaries, and tiny primaries for good low end torque, driveabilty and gas mileage. Stick with something in the 600-650 cfm range and you will be much more happy with the results.