Hi Mike
Depending on the application and year they moved the vacuum tap around on the manifold. Is the manifold off the engine? If so I would just drill and tap the manifold near the carb. Do not try this directly under the carb as the heat riser brings the exhaust gases up in that big area.
Another alternative is make an adapter plate for under the carb and tap the vacuum through the side of the adapter. This is how they tapped the vacuum for the brake booster on some of the engines.
Take a look at
http://www.canadianmilitarypattern.c...ifications.htm some vacuum tap locations.
Cheers Phil