According to this Apple Support page...
•How to install Safari 6 - Apple Support
you should probably reinstall Lion 10.7.5
Also, the MacPro Intel-tower is not a G5
PowerMac, unless you're running 10.5.8.
Then, it can't run Safari 6.1.
There was a page (in Support) that showed
all the downloads for certain Safari versions
and they were full. You may have to remove
old download receipts from former installation
to be able to re-install the same one again...
But it is gone, as are links to get them directly.
They aren't in the app store, for old versions
that cannot use it. So much for simplicity.
Some of this has changed as OS X evolves.
Or de-volved, as some may suggest! 😎