Jenvey throttle bodies on an Alfa V6 24V photos.

[Sando]

I was wondering, where do you pull a vacuum for the pressure gauge. Normally it would come from the intake, but I don't see anywhere on these throttle bodies to attach a vacuum line.

Not quite sure about vacuum for a pressure gauge?
But I drilled and tapped just one inlet runner for the servo (booster) take off. Works just Fine. No issues at all, ( it was always common practice for multi carb set ups back in the day too) it does run through a standard non return valve, but you would be using that wth any servo take off.
If you needed vacuum for anything else, I'm sure it would work just as well, however you would most likely see the pulses on a guage unless you dampened it out. (A piece of threaded rod in the hose may do the trick...Early Turbo boost fooling trick to bleed pressure or vacuum slowly)
Rob