Go to Holley's website and use their carburetor selector. One thing you will note is the criteria for selecting mechanical or vacuum secondaries. Frankly, unless you spend a lot of time on at the dragstrip, the VS carb is a better choice.
They do list a 650 VS carb: PART #: 0-80783C. I'm not why it doesn't show up but the Avenger 670cfm would work as well. The 750 Vacuum secondary is really not as big as you think (also shows up as an option) and would still work well with your 400.
As Bjorn says, the best carb is the one you (or a friend) knows how to properly tune. That being said, ask your mechanic if he'd be willing to help out with a Holley with VS rather than a DP.
