Chris,
At the manifold end of the carb, you should find a sloted screw, fitted vertically on the LH side of each carb. This is the vacuum take-off, has an M5 thread. Remove the screws, fit a couple of hose barbs and your vacuum guages. The bracket connecting the carb tops has appropriate bores through which a screwdriver can be inserted to facilitate removal of the screws.
The barbs can be left fitted to the carbs and either blanked off seperately or connected with a length of hose, as on the later triples.
piet