What is this and why doeant my aluminum manifold have the space for it. It has the two bolt holes but no space for the sensor "thing"
Aluminum manifold
It is called a choke tube and is a heat tube for the carburetor to adjust the choke to running temperature.
The new aluminum intake does not have it because the assumption is that you will be changing to a "performance" carburetor that will use either a manual or an electric choke.
The two holes are still necessary to bolt down the intake manifold.
625 CFM sounds about right for sizing on a basically stock motor.
Carburetor make is up to preference for the most part. I personally like Holley (now I will duck) while others really like the Carter and Carter type makes. I had a Carter on a 460 in a F-150 and it was a great carburetor. I could not fault it.
There are more qualified opinions here on the forum when the talk turns to carburetors and carburetor tuning. We will see what they have to say.