Both works.
Found this in an amc forum:
This is a controversial topic in some circles. The factory used ported vacuum. I.e. the factory used the carb's spark port. In off-idle conditions there is actually no difference between the two. The difference is just at idle. At idle you have no vacuum advance with ported vaccum. That's because the spark port is just above the throttle blades. So when the throttle is closed, the spark port sees no vacuum. However, as soon as you open the throttle, even just a little, the spark port sees full manifold vaccum. With a carb manifold port, the port is just below the thottle blades so that port sees full manifold vacuum at all times. As far as how your engine runs, with ported vacuum the distributor supplies initial mechanical advance only, at idle. With manifold vacuum, the distributor supplies intial plus full vacuum advance at idle. Which should you use? Well the factory always used ported vacuum and Holley and MSD reccomend using ported vacuum too, so if for no other reason I'd say go with ported vacuum at least with a typical street motor that pulls good vacuum at idle. With a really big cam that doesn't make much vacuum, it probably doesn't make any differnce.
Just to be a little more specific. The factory actually used both manifod and ported vacuum (at least during the '70s). If look at a TSM showing the vacuum hose routing, you will see that the factory used a spark CTO (coolant temperature overide switch) in the thermostat water outlet. There were two vacuum lines into the CTO. One supplied manifold vacuum and one supplied ported vacuum. A third line ran from the CTO to the distributor vacuum advance. What the spark CTO did was supply full manifold vacuum to the vacuum advance upon startup and until the CTO reached 160 degrees. Once the CTO reached 160 degrees, the CTO switched from manifold to ported vacuum and supplied ported vacuum to the distributor vacuum advance thereafter. The factory did this because the extra advance at startup makes the engine run hotter and hence warms it up quicker.