Vacuum gauge connection to AFB

For reading the vacuum while adjusting for best idle with the mixture screws on the AFB, which is the best connection? I've always used the vacuum advance port, but I think that's not the finest reading because it's above the throttle plates. If I put a tee connection in the PCV line, would that be good enough to get a real good reading, or is a tee in the brake booster hose that goes directly to the manifold in front of the carb the best method? Other than the vacuum advance port, those are the only two places I can think of reading for vacuum. This is an original 1967 343 motor.

There is also a port on the back of the manifold near the firewall that is used for the automatic transmission. That should work.

Should do. Hose might be a bit big, but as long as that works you should be fine. 

On my 67 290 there is a square plug in the manifold in front of the carb, I have a brass fitting with the same threads that I put in to read intake vacuum. Purchase a rubber cap and leave it in when finished or pull the brass fitting and replace the with square plug.