The distributor vacuum line does not get full manifold vacuum- - - - - -little if any at idle, and only partial vacuum during acceleration and/or cruise. Just put a tee fitting into the port directly below the carburetor and run the line through any available hole in the firewall. The closer to the carb, the more accurate and responsive the reading will be. Any load at all such as acceleration or passing that requires the throttle to be opened will kill most of the vacuum reading.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!