Not sure why you'd need a T-fitting....that line hooks to each carb bowl, and has a port at the hex shaped inlet block for the gauge.
You'd still need a regulator to go with this.
Although I can't see all of it in the pictures you've posted, it appears you already have a good braided dual feed line on the Demon carb. You don't need a different feed line unless you just want to change it.
All you need to do it split the line from the pump to the dual feed line somewhere before the dual feed line connection, add in the correct fittings and the pressure regulator, and put a pressure gauge in the proper port on the regulator.