If you have decent brakes dig into the engine. Lots of different reasons for a backfire, could have broken valve spring, bent valve, burned plug wire, fouled plug etc etc.

Front brakes don't need to be costly. So many of these cars are getting aftermarket discs, that there are sets of complete, original, '69 disc brakes for much less money on craigslist and at swap meets. You certainly don't need $1,500 Wilwoods or other such to get the car to stop better than the drums. I have '69 Camaro D50 disc brakes on the front of my '66 Chevelle that work just fine to haul a heavier car than a Camaro down from speed.

As for the engine, it doesn't cost anything to pull the valve covers and look for a loose rocker arm that would indicate a cam lobe failure. A close inspection of the plug wires to determine if one, or more, has become brittle and cracked from age/heat and could be allowing the spark to jump to ground or to another plug wire and fire a cylinder out of order. Just making certain the wires are in the correct order on the cap and to the related cylinder. A very common mistake is crossing #'s 5 and 7. People do that all the time.
If unsure on any of this, take the people offering help up on that and let them. As others have said, letting the car sit for a period of time, just makes it more difficult to find the time to work on it. This isn't just a fable people throw around, there are THOUSANDS of us who have made that mistake and before you know it, 20-30 years have flown by and it is still sitting with flat tires and surface rust all over it.
I am very familiar with not having the funds to do much at one time, but, for now, your time is cheap/free and doing anything on it will keep the interest and desire level high.

I will say this, I watched one rocker arm (exhaust on number 8) barely move last year. Didn't matter how it was adjusted. So there's something internal, I'm almost sure of it. While I can't speak for the valves and don't know how to identify a bad valve spring, the pushrods themselves look perfect. We must also remember it was, as I like to say, knocking like a Jehovah's witness before the backfires started. You can hear it very well in this old video (Before you knock my head for revving the piss out of it- Pappaw did that, not me. His philosophy was that there was nothing that couldn't be fixed so I imagine he figured if it blew right then and there, well, we could fix it.)

https://www.youtube.com/watch?v=jOTsSMd4dhI

I've checked the plug wires they're new as are the plugs (though I probably should pull the plug and see how it looks- might do that in a minute here), firing order is definitely right.

As far as the brakes, it was the Right Stuff Detailing kit I was eyeing. Nothing super fancy for it, it doesn't need it.

Sounded okay to me, hard to tell. I'd have to listen again with earbuds, maybe it had a miss? I'd be more concerned about the blue smoke, that can be a few things.

You can really hear it right at the end with earbuds, it's *loud*

EDIT: Okay, what the heck? I swear you could hear it better than that in the video. Let me see if I have another

Here, when I go under the hood. It's only gotten louder since this

https://www.youtube.com/watch?v=P_LE24I8ceA&t=9s

Don't know if it tells anything, but here's both the #8 plug and #1

It's worth noting they have maybe an overall hour of runtime and 2 miles on them. They went in in 2020 but the car hasn't been started but a handful of times since.

I can't tell (lack of experience here) if they're oil fouled or just carbon fouled

https://youtu.be/6eXGUYQU5FA
Finally, this is the best video I have showing the problem.

You can hear the noise, and see how #8 doesn't travel as much as the others. And it wasn't any looser or tighter, either.

I can't get it to embed properly.

I'd change those plugs for sure. I personally hear a backfire though then you went to the front of the car I hear a tapping which could also be an exhaust leak from the headers.

There does not seem to be much oil at all on the top end (by what I can see)

And yes, #8 exhaust has minimal movement.

It had only been running for a few seconds in that video. It gets oil okay thankfully, and if the gauge is any good (and I probably should put another one on it to verify) it cranks at 60psi.

So there is absolutely an issue with the #8 cylinder, it's just a question of lifter, cam lobe, both, valve, valve spring.... etc.