I have checked out the Xerox.com/support Drivers & Downloads page for the Phaser 7400 and I see a similar driver version to the one that you mentioned in your post. The version on Xerox.com is 5.50.30 and is dated Jan. 27th, 2008. There is a PostScript driver installer file and a separate Post Script driver file both with that version number and release date. I don't know if you downloaded the driver file from Xerox.com/support or not, but if you didn't, you might want to try the two files that I mentioned above. Failing that, there is a Global Print Driver available that was just added recently (February 2011). That might work for you.
We're using the latest 7400DN drivers (126.96.36.199N from 2008.01.10), and I have a weird issue: with this driver, I'm missing the "ouput destination" field
Weirdest: it works with the old pcl driver (one dated from 2005, but that one is missing other options, so we can't use it either).