Because we're talking about a '74 here....
It comes out of the base of the intake through a hole on the heat shield.
when the car left the factory, that port operates the diverter valve for the air injection system to shut off the air injection when you lift off the throttle. If you no longer have a smog pump you should just cap it off (although it is also by far the most convenient place to hook up a vacuum gauge when you want to do that).
I don't believe 74s had an egr valve from what I've read.
You are right, they did not. Started with the '79 carbed 1500.
Anyway, I checked the vacuum advance, and it's working correctly. I set the timing to 0 by the factory service manual and it already seems to be running much better.
How exactly did you check the vacuum advance? Look carefully to see whether applying vacuum is advancing or retarding the timing. This is because in stock trim the '74 has a vacuum retard, not advance. The shop manual procedure is to disconnect the vacuum line and set the timing to ten degrees advanced, connect the vacuum line and verify that timing goes to TDC as the vacuum retard kicks in.
If you have everything working and you like how it runs, you're cool - you have the car working the way it did when it came from the factory. However, I if you have no smog pump I find that it's easier to just cap off the vacuum line to the distributor, then set the timing by ear - you want as much static advance as possible without pinging at full throttle and peak revs uphill.
The logic behind the vacuum retard is that under the '74 regulations emissions were measured at idle, mainly because back then it wasn't practical to do anything else. Retarding the timing under conditions of high manifold vacuum, which is to say at idle, lowers combustion chamber temperatures and cuts back on NOx; back before catalytic converters there was no way of getting rid of NOx except to not produce it in the first place. This cooler and less complete combustion also produces more HC and CO, but the air injection pump takes care of these by burning them to CO2 and H20 in the exhaust manifold.