Timing can seem complicated, because there are a lot of factors. If you’re running smog equipment, it gets even more complicated because your engine will switch between ported and manifold vacuum for your vacuum advance.
For any street car with a mild cam, manifold vacuum works best.
Your mechanical timing is the baseline for your engine at Wide Open Throttle. It needs to be low enough that your engine won’t ping, but high enough to deliver crisp response. Typically, around 8-12 degrees is where it should be at idle. More importantly, you should end up around 34 degrees all-in by 2800 RPMs. The type of heads, and the cam you use can change both those values a bit. A 289 Hi-Po for example, prefers 38 degrees total timing, because of its high-lift mechanical cam, with a lot of overlap. The factory heads burn pretty slow. A typical build with a 5.0 HO cam and GT40P heads usually works best at around 28 degrees total timing. This is because the revised combustion chamber and mild HO cam result in a very fast-burn situation, and advancing the timing too much just means you’re hitting peak cylinder pressure too early, stealing power.
It’s important to understand that by setting your mechanical timing, you have vac advance disconnected and plugged. You are NOT tuning the engine to idle ‘smoothest’ when you’re setting your base (idle) mechanical timing. Remember, this is what your engine falls back to when you’re WOT.
Once you have it set, then you hook up the vacuum advance, and that should add another 16 degrees or so, when the throttle blades are closed, using manifold vacuum. When there’s strong vacuum in the manifold, there is very little air and fuel to compress in the chamber, so pressures are low. Because of these factors, the flame travel is very slow, and the explosion happens very slowly. Lighting things off 16 degrees sooner than you would at WOT, you’ll still reach peak pressure at the proper time to push the piston down, instead of the explosion chasing the piston down the bore, and spitting flames out the exhaust.
Using timed or ported vacuum turns off vac advance when the throttle blades are closed. It was used in early smog systems to ensure that there were flames in the manifolds to burn with extra air from the air injection system, and later to heat the catalytic converters. But ported vacuum causes underhood temps to be much higher (from the much hotter exhaust manifolds), and causes some drivability and tuning issues.
Using those values will get you in the neighborhood, but only driving the car and testing it (when the car’s engine is warmed up!) will tell you if it’s set perfectly or not. Once your timing’s set right, then you can work on fine-tuning the fuel system. If you have a choice, use a carb with annular boosters. They are flat-out superior for street use.
So: TL;DR: Use manifold vacuum. Mechanical with stock early heads should be about 10 BTDC, and around 34 total mechanical by 2800.