Hi there, and welcome to the JBoss forums.

As you have seen, the JBoss AS 7 source is available on github. In order to get a specific version you need to checkout the associated tag.

There are also a number of supporting projects, most of which are listed at AS7 projects. You may also find the information at Hacking on AS7 useful.

Finally, you will need to get up to speed on git-scm.

You need to learn how to checkout a specific tag from a git repository.

The JBoss git repository has a tag for every released version. Choosing which one is really up to you, although you might consider using 7.1.3.Final.