I have a vacuum gauge installed on the dash of my '59. I used the same port on the intake manifold where the vacuum windshield wipers got their vacuum.
http://www.flickr.com/photos/11159813@N00/151949993/
When using a vacuum gauge you want a reading from a place in the intake manifold where you will get a good average reading from all of the cylinders. You could easily hook up a temporary gauge in this location.
On a stock engine you should have a constant reading somewhere between 18-21 in. Hg. at idle. If the carb. is adjusted properly the needle will not move at all. If the mixture is rich or lean there will be an irregular drop in the reading. A vacuum gauge can help solve other problems as well.
Here is a web site that might help you out also.
http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm Good Luck
