I have a 642 with the ford motor. The only vacuum line goes to the vacuum advance on the distributor. Can't recall where it connects to the motor, but can check when I get back home...
I can't tell you for sure. But the vacuum line should connect to the carburetor. On a distributor it should have the diaphragm vacuum advance. There should be a nipple or a connector there.
Good luck.
I just checked my 642 and the distributor vacuum advance hooks to a fitting at the top part of the intake manifold. Right by the larger hose. I hope this helps