OK first off, I know you know stuff, but I will word this like you don't know anything. You never know who is reading this and wants to learn. You have the base timing set but let's expand on that. The engine needs a small amount of timing advance even at idle. Some engines use 0° and others use up to 16°. Every engine and situation is different so we go with what the factory wants. Your 1970 350 is 9° BTDC. At idle with no vacuum or centrifugal advance that is what will show up on the balancer with a timing light. The distributor has two more timing advance mechanisms. Under the rotor there is a centrifugal advance using weights and springs to hold them in. As the rpm increases, centrifugal force overcomes the springs and throws out the weights which advance the timing. This timing advance curve is there for the entire rpm range. These weights and springs are able to be changed to get different timing curves but we will stick with what you have. Then on the side of the distributor is a vacuum advance canister. Vacuum applied to this will also advance the timing. A rubber hose connects the canister with a port on the carburetor. Here is where it gets tricky. Some engines like ported vacuum, and some like manifold vacuum. Manifold vacuum is the high vacuum that exists UNDER the closed throttle plates while the engine is running. A normal stock engine should have between 18 and 20 inches or more of manifold vacuum at idle. As the throttle opens, this vacuum drops. Ported vacuum is taken from ABOVE the throttle plates. So at idle there is NO ported vacuum. As the throttle plates open the air velocity increases and ported vacuum increases. Older, high compression engines liked manifold vacuum. It advanced the timing at idle and smoothed it out. As rpm increases, all that timing advance would be bad and cause detonation. But as the throttle opens, manifold vacuum drops, and the extra timing goes away. Eventually at higher rpm the manifold vacuum increases but at that point the engine is spinning fast enough to use more timing. Engines with a big lumpy cam suffer from low manifold vacuum and the idle can be very rough. By using manifold vacuum hooked to the vacuum advance, the extra timing speeds the engine up and builds more manifold vacuum and smooths out the idle. Ported vacuum is usually used on a stock low compression engine. This type engine has good manifold vacuum and a smooth idle. As the throttle is opened the ported vacuum rises and adds timing which the engine can use to make power. Your 1970 engine has 8.8 to 1 compression. While considered low compression in 1970, later smog engines would go as low as 7.6. So you are actually in the middle. I would test out both ported and manifold vacuum and see which one worked best for your particular engine. If you have a nice smooth idle I would expect ported vacuum to work well. On a Quadrajet carburetor, the ported vacuum port is usually on the front, down low, on the driver side of the fuel line. Sometimes it is on the passenger side. Check with a vacuum gauge to be sure. Almost all the other vacuum ports on the carb and intake manifold will be full manifold vacuum. Next we will address the carburetor.