How to install fuel press. gauge on a 740 GL?

I have an intmt. no-start problem with a 1991 740 GL turbo wagon.
Rotates OK, but sometimes will not start. The spark plugs are firing
OK. You go back the next day, and sometimes starts right up. I think
it may be that the fuel pump is not working intmt. I think there is a
fuel pump relay behind the ashtray, I guess I should check that, see
if whatever wire is the power supply to the pump is hot. Also, I have
a fuel pressure gauge and would like to test the pressure when car is
not starting, if the fuel pump is running. Problem is, there is no
schrader tee on the fuel rail, so I cannot connect it in. I suppose it
could either be one that connects to the flare connection at the rear
of the rail For this, the tee would have to be female on the rail
end, and male on the fuel supply end. Or I could get a tee with a
schrader valve that is barbed on both sides and install it on the
rubber line that cannects to the inlet /rear side of the rail .
Either way I go, I can't readily tell what the metric sizes for a tee
for the first option are, so that I can ask for the right thing at the
store. And for the second--- the hose tee---I don't know what the ID
of the hose is, so can't tell them exactly what I need for that,
either.
Anyone know what the thread or barb sizes are for a tee like I need?
Is there maybe a standard Volvo part I can ask for?