Well nuts - I wrote a "genius post" and then lost it because I cleared cookies in the middle of it and had to log back in. It was great, Pulitzer Prize worthy - trust me
There's a guy, David Vizard, who's a real expert on the topics of tuning, air flow dynamics, carburetors and induction systems. He's written and published quite a few books on the topic including one specific to Holley that I recently have been reading to try to learn about them.
According to him, the way to estimate what size carb is needed is to use the formula: CFM = (Max RPM X Engine size X "correction factor") / 3456
You have to be honest and realistic about max rpm. Don't use 8000 rpm for a Pontiac street engine that in reality is never likely to spin past 5500. Just like it's possible for a carb to be too small and 'starve' the engine, it's possible for one to be too big. Carbs depend on a certain minimum air flow velocity to work right, and if one is "too big" for the engine then air flow velocity through it won't be high enough to make it work correctly.
That "correction factor" is the hardest to pick because it depends on the details of the engine - cam duration, head flow, manifold type, etc. all things that effect volumetric efficiency at various rpm. The factor ranges from 1 on a 'theoretically perfect' engine up to 1.11 on one with a really long duration race only cam and other similar mods, at least that's what's in the chart in his book I'm reading.
Let's take your +0.030 400 (works out to be close to 406 inches) and see what comes out of the formula.
At 5500 rpm with no correction factor (1) it works out to 646 cfm needed
At 6000 rpm, 704 cfm
At 6000 rpm, correction factor of 1.01: 712 cfm - 1.02: 719 cfm - 1.03: 726 cfm ----etc
Also keep in mind that Holley 750 isn't going to be passing an actual 750 cfm on a running engine. Carb manufacturers give cfm ratings that are frequently dry flow (air only) numbers instead of wet flow (with fuel) numbers which will always be lower than dry flow numbers, and also with a vacuum "pull" on the carb that's different than what an engine generates. Recall at WOT manifold vacuum is very very low (not zero because there has to be some vacuum present to make it work). However, if a carb vendor happens to measure CFM on their products by 'sucking on them' with 10 inches of mercury vacuum pull, then technically they're telling the truth, but your engine will never see those numbers because at WOT it's only making 1 inch of mercury vacuum - or less.
Based on the above and your "seat of the pants" experience I'd bet that your engine will be very happy with a "750". Also food for thought: Every Rochester QJet can flow 750 CFM. Some, like the 455 SD version and a few others, can flow 800---- just sayin'