>expensive to buy a gauge that's already set up that way,

Here are some absolute pressure gauges to admire...
http://www.ebay.com/sch/i.html?_nkw=absolute+pressure+gauge&_sacat=0&_odkw=absolute+pressure&_osacat=0&_from=R40

And a nice differential pressure gauge that would do beautifully for
the fuel pressure regulator - the Buna N diaphragm is resistant to
gasoline.  One line to the fuel tee, the other to the manifold, Bob's
your uncle.
http://www.grainger.com/Grainger/MIDWEST-INSTRUMENT-Differential-Pressure-3GVH4

Yours,
David