Excuse my ignorance, but when you say port, are you talking about carb?
I have a regular distributor and my vacuum advance is connected to my carb at the base.
67 GTO 400 block, Dominator carb.
Should I have the vacuum advance connected at the manifold then?
Port would be on the carb, which only provides a vacuum signal at part throttle. Manifold vacuum is found at of course the manifold, and/or at the base of the carb. Simply check the port with your finger when the engine is at a closed throttle idle. Some of the carb ports have vacuum at various throttle positions. Best way to check is buy an inexpensive vacuum gauge, which is a high value tool when tuning these cars!
There are different schools of thought on port vs. manifold vacuum for the dist advance. Most would say manifold, however in the Cliff Rugles qjet book, he leans towards ported vacuum, and performance tests did not show an advantage with either source. I have tried both on my "mid-performance" 455, and really cannot tell much difference, although it will idle better with manifold vacuum.
Do a search for the tech article "tuning your gto" or something like that, which was posted by LARS, it has some good information.
STEP #1 -- Buy yourself an adjustable timing light, and FORGET about initial timing settings at idle. Only concern yourself with total mechanical advance with the vacuum canister disconnected, should be 34-36 degrees. AND, total advance with FULL vacuum advance connected, (i.e. hold the throttle at steady 3,000 rpm while checking total advance) which will be in the high 40's or low 50's. As long as the engine turns over and starts ok, your initial timing is fine. If you are getting pinging or knock at any RPM, you either need to run higher octane fuel, or back your timing off a little.
It makes sense that car-guys with limited experience are concerned about initial timing, as that is how the factory sets it up, but how many of these cars are actually "factory" anymore? Almost none. The above process is really easy to get the hang of with just a little reading & practice.