I have a '75 400F currently going back together (very slowly!) and will soon need to calibrate mycarb bank.
I found a set of vacuum gauges on Ebay UK for (IIRC) just over UKĀ£30 (US$53) - there are four large face gauges, two short and two long brasstubes to fit the take-off points on the carbs, appropriate rubber tubing (connecting the brass tubes and the gauges, and tee-piece unions. The kit was (of course!) made in Taiwan, but the instructions are all in good English - the whole thing is neat and appears easy to use - I'll let you know when I attack my carb bank!
However, I haven't seen a repeat of these on Ebay for some times, but it may be worth doing frequent checks in case they turn up again.
Seabird