Some engines use ported vacuum and some use manifold vacuum. I am running a 78 305 in my truck, with 2 GC carb also. My engine uses manifold vacuum as per the factory set up. However, the orignal car had a vacuum switch that limited vacuum until high gear. A pollution lowering device. My carb also has a smaller ported vacuum below the manifold port.
I have a 79 Chevy Caprice at school with the same engine and set up.

I am running my truck at 8 degrees BTDC and using the manifold vacuum port. I get a slight ping on Heavy accel, but that is all.