Pacemaker is not supported on Red Hat until revision 6.5. If you're not worried about support, and don't have access to a revision of Red Hat higher than 6.3, I would recommend CentOS 6.5 or 6.6. This would be the easiest route.
If you absolutely need the 6.3 revision, you'll have to do your best with 3rd party RPMs and/or repositories. I would probably try to add a CentOS repository
http://mirror.centos.org/centos/6.6/os/x86_64/, or just download the packages manually if you know all the dependencies.