I would think a 600 would be a little small for that motor, the 670 sounds like a good choice. Those 600 eddy's are pretty good for mileage though snd I would be interested in it if you decide to sell it.
I just replaced my 580 street avenger with a 780 quickfuel on my 383 with a performer manifold - HUGE difference. The engine can get a full breath. I'm going to have the jets dialed in next month, but in my experience something in the 750-800 range really gets the air in and the fuel economy improved.
it depends on size of motor and rpm's that your engine runs,i use the holley manual .it says 350 ci turning 6500 use a 600 dp 7000rpm use a 650dp 7500 use a 700 dp,however i dont think a 400 horse 350 needs to windall the way to 6500 so id say a 650 dp would do great so youll have some head room
That size carb should work well on your engine. If your looking to get out of it what it should be making take it to a dyno shop for tuning. On Dereks Comet they found annother 40 rear wheel HP. I think it was SVS in Sac. Look here.