My 2004 6.0L needs a "T" fitting to allow the factory "dumb" gauge to be installed and an aftermarket 0-100 PSI gauge to be installed.
Car Quest has a T that will work. Simply unscrew the sendor and screw the T in and then install the factory Sendor in the female M12 hole and there is another Female for the 1/8" NPT Sendor for the aftermarket gauge.
The part number is CPS 81639 and at my Car Quest is $12.
For those of you not in the U.S. LMA052 is the part number.
This link has a pic of it as well. http://www.speedograph-richfield.com...ipe_lines.html
Hope this helps someone.