Jeremy, what you're talking about is very consistent with ported vacuum instead of manifold vacuum. Edelbrock suggests you use ported vacuum, which might be appropriate if you have a really hairy cam. Unless you have a really surging idle though, using manifold vacuum makes more sense.
Here are a few basic points to help you figure this out.
1) At idle, and even at cruise, your engine needs a LOT of timing in order to make peak pressure at the right time, because the thin mix (high manifold vacuum) takes a long time to light. Having a total of around 24 degrees with vac advance for idle is pretty common.
2) With the throttle wide open, running near atmospheric pressure instead of vacuum, you need a lot less timing to avoid ping/detonation. (14 could be all right, but it's towards the high end)
3) Your final timing is most important, because it is more likely that your engine will blow itself apart with detonation while making max power, instead of at idle.
You're using vac advance, which is good, but you're using ported vacuum, which means when the throttle plates close, you have no vac advance. Using manifold vacuum makes more sense, because you have full vac advance at idle, allowing for a smoother idle and less unburnt fuel in the exhaust. You get more advance when your engine needs it (high vacuum) and less when you don't (no vacuum). You'll have smoother off-idle response when you step on the gas too.
After reading your reply to my question I had a dig into my carburettor and found a problem. You said that what I am experiencing is consistent with running the distributor off of the ported vacuum connector and not the manifold vacuum connector. Well, I actually have it running off of the manifold vacuum connector.
I found that the small manifold vacuum connector had a partial blockage inside the carburettor body and it was acting like the ported vacuum and had no vacuum at idle and only gave a vacuum when coming off the idle. When I blew through it, it felt restricted. So I decided to strip the carburettor down and clean it with carb cleaner everywhere. After the cleaning, I could now blow through the manifold vacuum connector easily.
I set the idle speed such that the transfer slots were only showing a little square of the rectangle which seems to be the recommendation. And the idle mixture screws were set to 2 turns out from seated. Once the engine was up to temperature I set the initial timing to 12 degrees with the vacuum disconnected and plugged. I could feel a vacuum on the manifold vacuum connector at idle which I could not before, so it looks like this is now working as it should.
I set the idle mixture screws and got a vacuum reading of 10 inches and an idle speed of about 750 RPM. I reset the plate in the distributor to ensure the total timing did not go above 34 degrees. And the springs stayed the same which brings in total timing by about 3000 RPM. With all of this work, the idle is very smooth and I now have things set up in line with Edelbrock's recommendations, which makes me happier.
When driving the car, fast acceleration and wide open throttle seemed about the same as before, but there is a big improvement when driving slowly in different gears. Before, if I slowed down too much in 2nd, 3rd, 4th or 5th, it did not like it at all. Now I can slow right down in 3rd and just pressing the accelerator it wants to pick up speed easily. And 2nd gear can easily go down to a near enough standstill and not complain. I can even pull away from a standstill in 2nd with ease. So it looks like I have gained some low down drivability and this ties in with the manifold vacuum connector now working correctly.
So thanks for your reply which has enabled me to home in on my issue. I would like to take the car to a dyno, but as I mentioned in another reply above, there are not many around my way and I cannot really afford it anyway. I have no complaints about how it drives now. Also, another problem with dyno's these days is finding one that knows about carburettors.