Backfiring and I'm trying to figure out why

On a carbureted engine this is caused by too much fuel. Most all backfires are caused by too much fuel but on modern engines it could be caused by a faulty air flow sensor making it think it is lean, so it dumps fuel. Here is a guide that talks about a modern engine, but the idea is the same, so it is worth sharing:

https://www.2carpros.com/articles/engine-backfires-while-running

How this applies to your engine is the same idea but without all the sensors.

So, you have a carb that determines how much fuel to dump based on throttle input and that is added to the air that is rushing past the nozzles.

When this air fuel mixture is not correct, you get a rich or lean condition each with their own issues. Rich conditions foul spark plugs and can cause back fires.

The first thing to do is checking the total timing. You do this by checking your base timing and then accelerate to about 2,000-2,500 RPMs and see what the advance goes up too. What I suspect is that it is too far advanced, so you need to back your base timing down to around 10 degrees for the 289.

If this is not the issue, then I suspect you have an issue in the carb on the secondary side where it is leaking too much fuel.

However, based on the fact that the you only have this at higher RPMs makes me think we have a timing issue. If you adjust the timing to 10 degrees and the advance is still higher then 30 degrees, then you need to change the springs in the distributor so that it doesn't allow it to advance as much under the same base timing setting.

Let us know what questions you have, and we can go from there. Thanks