Just to be clear, 20° base timing, was this measured with vacuum advance port disconnected and plugged?

One misconception about the standard carburetor vacuum advance port, it is popularly supposed there is "no vacuum at idle".

Technically that's true but only provided that the idle RPM is adjusted to factory spec.

Most people seem to bump the idle up to say, 600, 650 or more. Guess what happens? It starts to pull in lots of vacuum advance, smooths out the idle, and so it isn't unusual at all to see numbers like 20° or more at idle. It's also possible that if light springs are installed inside the distributor, they will start to add some mechanical ignition advance too, if the idle RPM starts getting up there.

Just to be sure, when setting the initial timing, always drop the idle RPM down to a crawl, disconnect vac advance, for accuracy.

For all that, 20° or 30° of timing at idle won't hurt a thing, many people prefer this, and will tend to run much cooler in stop and go traffic. It won't pass smog setup like this however.

 

Thankfully I don't have to pass smog

Timing has been taken with the vacuum advance port / ported vacuum disconnected and plugged on both the carburetor and distributor.

FE motors are not something I've worked on until now so the nuances of them are new to me. I'll fiddle around with timing some more in the coming days/weeks and see what various changes net me performance and MPG wise.

I think I also need to dial in the carb some more.

 

Right, but retarding the base timing shifts whatever the total mechanical advance was before at the far end by an amount equal to however much you took out.

So, what was 36° at 2800 RPM is now say, 29° (or whatever.) No Bueno.

34°, 36°, 38° (This would be without vacuum advance) is right in there for practically every OHV V8 ever made. The compression ratio, fuel quality, combustion chamber design etc etc, determines the optimum amount of advance, but the idea is always the point of maximum ignition advance at all times under all conditions just short of engine knock or detonation. This is true whether a drag racing engine, grocery getter, weed wacker. It's also important that the ignition, and ignition advance curve, is sussed out thoroughly before diving into the final carburetor calibration tuning AFR or mapping out EFI.

20° is pretty stout for an initial setting but if there is no starter kickback, i.e. it starts easily when hot, it isn't a technical issue in any way. There's no load on the engine in neutral, and it will run cooler in hot weather as well. Might consider recurving the distributor if it's an issue, to get back to the 36°.

The manifold vacuum fluctuations are something else, that needs to be investigated.