On a different message I was told to use a 750 carb on my 340. I plan to get the Edelbrock performer RPM air gap dual plane intake. Which carb should I use? This is my first car so I am learning as I go. All I have worked on was EFI ford trucks. Also, how much difference will a spacer make?  

Spacer will make little different, get the 750 holley double pumper, you will pay more but it is worth every penny.  

I never heard anyone have good luck with a 750 edelbrock. I tried one and switched right back to the 600. I later tried a 650 double pumper which worked even better and eventually modified that with a set of Proform metering blocks and a Proform main body which flows about 830cfm. This was a pretty mild 340, small cam (have no clue what size it was) LD340 intake , header and 391 gears.  

Weak heads?
Most factory heads don't flow enough.
Because the heads don't flow enough cfm for the cubes, the flow velocity is so great they tend to like bigger carbs.

Most edlebroke carbs are too lean right out of the box, so you need a rod/jet kit and you need to know how to tune!

The holley is a better carb IMO anyways so f it.  

I have a friend who is gonna help me tune it. I plan on getting the Edelbrock performer RPM heads. I'm also gonna use flowmaster exhaust but I haven't picked out the headers yet.  

i started with an edelbrock changed to a holly and now run a demon. for the money you throw down on a double pumper you can get a "holly" based demon or proform carb with alot of upgrades. just something to think about. i also use a 1" spacer.