Backfire through carb among other issues

[68C101775]

Good evening everyone looking for some advice,

Going through the process on getting my truck driveable while starting on my first engine build and running into some problems.

First the details of the truck its a 1968 long bed, 2wd, 3 speed w a 250 L6 and a rochester monojet carb/manual choke.

I have rebuilt the carb and set the float to what the manual calls for and set the idle mixture screw to 2 turns out from fully seated. I replaced the distributor cap, rotor button, plugs, plug wires, fuel pump, and all new intake/exhaust manifold gaskets. After resealing everything I have sprayed all around with brake clean and don't think I have anymore major vacuum leaks.

I set the timing with the vacuum advance plugged off to 4 degrees BTDC

I attempted to set the idle mixture screw with a vacuum gauge but when I turn it either direction it doesn't seem to change the vacuum a measurable amount so I've left it at 2 turns out from seated.

I am chasing several problems but one big one is when I start the truck and rev it up in neutral It backfires badly through the carb almost every time I rev it. This does not seem to happen with the choke pulled out about 50% or more.

My other problems include dropping way down below the normal idle speed after being revved and while driving it bogs way down and sometimes dies while I am braking (no vacuum on the brakes). Any advice is appreciated.