My opinions on your question.

1. Vacuum secondary or air door secondary called for in your application. This means Holley vac secondary or Edelbrock either AFB or AVS. Of these, I like Holleys because I grew up with them.
2. I agree that this engine will work fine with about a 750 cfm carb. But, unless you're going to be making more than 400hp a 670 will work as well. For example the 400 in my $500 truck used an R-1850 600 cfm carb and ran strong.
3. I wouldn't put too much emphasis on the quick change vacuum secondary spring cover. It's a "nice to have" feature but unless you're racing you most likely won't use it. The carbs come pretty well set up that way, especially if your engine has decent vacuum.

My personal choice for you is a 670 Holley Street Avenger. You did say mild 400, right? You can buy them for a little more than $100 on the 'bay. They come standard with the power valve protection and the quick change vacuum secondary cover. Most of them have electric chokes.
Holley has done a fine job on the Street Avengers, my 570 runs so much better than the 1850 it replaced, there's no comparison. It's a 4150 so it has metering blocks with replaceable jets front and rear.

Next down on the list is the Holley 80508. It's the modern version of the 3310. It will have power valve protection but no quick-change vac sec spring cap. I believe that Holley has gotten better with the "out-of-the-box" tune and I wouldn't be surprised if the 80508 smokes the 3310 in terms of mixture correctness and mileage. One last thing, the 80508 is a 4160 carb meaning it has a secondary metering plate instead of jets. That's not as bad as it sounds as most jet changes are on the primary side. AND, one could always add a secondary metering block.

In third place is the venerable 3310, 750 cfm Model 4160. This carb first saw the light of day in something like 1968. Something tells me it was on the 375 hp 396. The first two versions had downleg boosters, the first version rated at 780 cfm and was a Model 4150. From -2 and on, it has had straight leg boosters and is a Model 4160, rear metering plate, 750 cfm carb.
From your question I am assuming that you really don't know your way around a carb, and that's the 3310s downfall. Sure, you can still buy them new, but most of the used ones are pretty old and nearly every one I've seen has been used up or boogered up in some fashion. Choke plates gone, choke mechanism gone, weird jet sizes, etc.....A 3310 has much more chance of not having the power valve protection and it doesn't have the quick change spring top. Also, there's a much more likely chance it has been drag raced and that can mean all sorts of tuning that may or may not match your engine.

So go with the Street Avenger and never look back.

Carter-style carbs - If you find a screaming deal on a 750 Edelbrock, pass it by. But a screaming deal on an Edelbrock 800 would be great for your engine as well.

R.