1987 Caballero - Fuel pressure test tee fitting at TBI threads?

[akahan]

1987 GMC Caballero 4.3L V6 TBI
I want to check the fuel pressure at the TBI inlet.
The Haynes manual says, "Disconnect the fuel feed hose at the inlet fitting on the TBI unit and attach the fuel pressure gauge inline with a T-fitting." Very helpful, but...

I am having a devil of a time finding out what is the right thread size for the tee fitting so that I can just unscrew the inlet line from the TBI, and put the tee fitting where the inlet line goes inline between the inlet line and the TBI.
I'm feeling like a dope, but what tee fitting, exactly, do I need for this?

For bonus points, who sells one? Thanks!

 

[383_StealthRam]

those are usually 18mm, or 16mm metric o-ring fittings, similar to the metric power steering box hose pressure and return captured o-rings.

https://www.harborfreight.com/basic-fuel-injection-service-kit-64939.html

I believe it's this fitting in the kit?