Probably the old standard 3310 Holley, it is 780 CFM far as I know.

Could be a number of issues.... a warped intake is possible, but not too likely. The FE's are not the easiest engine made to swap intakes on, so I would also be checking to see if one or both of the intake gaskets have slipped.... Not knowing any history on the engine also puts you at something of a disadvantage.... Also possible that the heads have been milled and is causing a misalignment problem on the intake. Should be fairly easy to determine the cause of the vacuum leak once you get it apart...