Base Timing Questions

It's all a balance. Large cam usually needs more idle timing to get a good carb tune and minimal rpm drop when put in gear with auto cars.

Too much initial and you'll get starter kickback. Vacuum and idle quality are usually better with more timing to a point.

More initial also tends to make off idle response better. Run an old 340 at tdc and then at 10-14 btdc for idle/initial timing. The way the engine runs will be very different.

Current tune doesn't sound bad at 20*, experiment with it up and down a couple degrees and see how it starts and reacts.