The easiest way to test the accessories connected to the carb and intake is to pinch hoses. 3 vac connections on the carb: one to PCV, one to vac advance, and one to auto trans. If you don't have three hoses connected find the bungs and close the one not connected.
I doubt a gasket leak is going to be this excessive.
On mine I had a similar problem. Eventually I figured out the trans line wasn't connected at the trans. Up top everything looked perfect. The trans line went from the carb, through a short rubber hose, to a hard line, down to the output shaft, through another rubber hose and into the transmission (powerglide). It was that final rubber hose that had issues.
Once I fixed that everything came together quickly. It only took me 6 months to find the problem. Only.
