The thread on the fitting i was supplied looks fine to be a standard metric, do you know if its a special thread?
Can you use the guages before connecting up the electric? i assume power is just for the lights.
The thread I welded/brazed in was a standard (1/4 npt I think) thread, it's the same thread on both ends of the T-piece, so yes, if you do that it's a simple case of unscrewing the boost gauge line, fitting the T-piece and screwing the boost gauge line into one end of the T-piece.
Tony