Red,
I did that in about 2001 or so. What I found was that the official "Racor" one ONLY reads vacuum. That is what you need to know, BUT, when we went to tropical waters, and everything in the engine room got HOT, it messed up the gauge. To much pressure from the thermal expansion.
I bought a vacuum/pressure gauge to replace the Racor in 2002 and it's been working fine ever since, hot and cold. Just a data point for you.
Greg
You can see it in the photo below.......