I had the same issue when I bought my Dart.

I bought the same gauge, and needed the same fitting to screw into the Stealth intake. Ended up having to go to a speed shop to find the fitting that the probe would fit properly into. The hardware store fittings that I found locally would screw into the intake, and the fitting from the gauge would screw into that one, but the probe would not seat. It would pass through the hardware store fitting.

The speedshop fitting was actually an Autometer brand fitting.

Today I installed a 440 source housing on my 493, and I needed yet another fitting to make the gauge work.

Never ends.

Hope this helps.

codfish