Adding to the confusion.

ALWAYS connect the vacuum gauge to the port below the carburetor on the intake manifold. Using the ports on the carburetor give incorrect readings.

I have not had good results using propane to check for intake leaks. I use carb cleaner or brake cleaner.

Use of a vacuum gauge for diagnostics. http://performance-diy.blogspot.com/2012/04/vacuum-gaugereading-and-interpretation.html

Using a vacuum gauge for timing & tuning. I quit using a timing light on these engines years ago. http://automotivemileposts.com/garage/v2n8.html

Fred
52 3600
69 C-10