I was a smog mechanic. We had the advantage of running the cars on the dyno while they were hooked up to an oscilloscope scope. It is strange that it has a misfire at lower rpm and then it goes away at upper rpm. One thing we would do is run the car up to the point it started to misfire on the dyno and then give it little shots of propane to richen the mixture and see if it straightened out. That was a way of determining if it was fuel or ignition. We used small propane bottles with a momentary pushbutton valve on the top of it. It was a State mandated tool for finding vacuum leaks during the smog tests. You could also extend the hose from inside the car to the carb and use it while you drove around to see if it had any effect while driving.
It's an expensive tool but it's really neat for finding vacuum leaks. It's a lot cleaner to spray propane around your carb looking for air leaks than it is to spray a liquid spray. Plus, you can use it while driving down the road. You might be able to make something up for a lot less and go for a ride, give it some propane and see it straightens out. If that don't straighten it out, then I would assume it's an ignition problem. You're at a huge disadvantage without a scope to watch the pattern.
https://www.toolpan.com/OTC-Tools-71...3wmyvqUfxT04Gx
Mart has a good idea, check the ignition advance throughout the rpm range.