Well, as for the mechanical secondaries, I can't tell you about that one.  I do know that Edlebrock says that most people over carb their motors and on their web site, not picking Edlebrock to push, but he sounds reasonable.  He says that most street rod engines are over carbed and at the most for BBC 454, BBF 460, a 750 cfm is the max recommended.  PSI is another issue and 5.5 to 6 psi is good pressure and shouldn't drop below 3 or 4 psi at WOT.  Now watch me get slammed for this info from all those Holley purist  out there.

Oh well, I know my 750 EB works great and was easy to tune.  Stays that way too.

basic carb FAQ: http://www.holley.com/TechService/FAQ.asp?category=Carburetor

well according to that tech board, a 650 vacuum should do my motor just fine, anybody got any thoughts on this?

What's all done to the motor? From testing on drag strips, and on boats, seems that overcarbing can hurt performance, as well as under carbing. Depending on cam, heads, and cubic inch can all change what carbs are needed

But, having too big of a carb will decrease the velocity of air going into the motor, which can hurt top end mph

To answer the mechanical secondary portion of your question, when you hammer down on a boat (any boat) it takes a while for the vacuum in the intake to "catch-up" which can really cause a lag in throttle response, not hesitate but to be slow out of the hole.

 As for cfm, thats trickier because what you are really searching for is a "perfect compromise" start small then go up not the other way around, Most of the boats I've worked on in the last 15 years were overcarbed with very few undercarbed. Basically CID, cam, valve size, intake manifold, exhaust system, overall rpm, fuel type, are all in that equation and I don't think my calculator has that button

With fuel prices the way they are, take the carb off your lawn mower!
Have fun!