How much is too much dual quad

That's a lot of fuel going in. Two 400's is just about right. (800 cfm)
Are you talking just the accelerator pumps???? It would only be the same as a 600 Double Pumper Holley, which would be too small for his combo. Carbs don't "dump" fuel into a motor, accept for the pumps. And the tuner controls the shots from the pumps. They are not superchargers or blowers or any form of forced induction. The engine "pulls" the air/fuel into it. It will only pull so much air and the tuner controls this and matches the fuel with jetting, power valves, vacuum secondary springs.....