Misfire at cruise, popping/afterfire at 3000RPM

Put a dial-back type timing lite on it and watch the timing mark. Do not concern yourself with the actual numbers.
Make sure the Idle-timing is relatively stable.
Next, slowly rev the engine up while watching the TDC mark. Quit when you get to 2500 or if you see the mark jumping all over the place from advance to retard and or missing or double strobes.
If this happens to you, the Pick-up polarity is reversed. Simply cut the wires and flip them.
After that, your base idle-timing will have to be reset.
Then repeat the test. You want to see the TDC mark move smoothly in the advance direction only, with no skipped sparks.

The hi-tension wires should NOT be bundled together. They should be kept at least 1/2 inch apart to prevent induction firing, AND the numbers 5 and 7 wires should NEVER run side by side in any case.

By the look of your plugs, your engine has got some serious fueling issues as well, but you have to clean the ignition up first.

Afterfires with open headers is sortof common. It has it's origins in a couple of places;
1) if it starts as a lean-misfire, then the unburned fuel charge moves down the primary, and gets lit up by the following exhaust cycle and it explodes in the pipe making the pop you hear.
2) if it starts out as a rich mixture that didn't find enough oxygen in the chamber,it exits into the pipe, and finding oxygen there, it finishes burning in the pipe and makes a weaker pop.
3) If the manifold to flange gasket is leaking, you are almost guaranteed to have afterfires as atmospheric pressure forces air into the pipes any time the pressure in the pipe is lower than atmosphere, which is most of the time.
4) If you have a long-overlap cam, idling very slow, the header puts a pull on the intake plenum, and tugs fresh fuel charge right across the top of the piston at the very beginning of the intake stroke. That air-fuel charge has the right A/F ratio to burn, so if something lights it off, you get a pop.
5) numbers 5 and 7 are consecutive firing (as are 4 and 3), so if #7 gets induction-fired at the same time that #5 is firing properly, but #7 is on the end of it's intake stroke just beginning it's compression stroke. , well you can imagine that the force that drives the piston back down the way it just came, is not good. Yeah yur gonna feel that too.
6) If one or more sparkplugs has a cracked center-electrode insulator, the spark will leak away, and not jump to the ground strap. So then it is possible for the A/F charge not to light and you get a misfire. But sometimes, the thing that caused the electrode to crack, is still in operation, and the A/F charge gets lit at the wrong time. If early, then it could drive the piston backwards. If late then the charge will not have enough time to complete burning in the chamber, so it enters the header still burning energetically. But more often than not, that charge goes straight into the exhaust unburned, and with a 4-cylinder you feel it as a major power loss.
7) if your cam-timing is off by more than say 8 degrees (all I have tried), all kinds of weirdchit can happen
8) kudus to the guys who mentioned valve problems, But I'd start with checking the pick-up polarity, and separating the plugwires.

IDK, that's all I can think of.
Happy HotRodding.

BTW
your engine, at idle, in Neutral/Park will like a lot of advance; I mean a lot. And it will tell you so by the fact that the rpm keeps going up, along with the vacuum.
If you had a timing computer, this would be fine, because as soon as you put it into gear, you could program the computer to get rid of it, cuz you can't drive it when it's been maxed out like that. It is impossible to build a STREET timing curve from that point, and you're engine is guaranteed to experience detonation.
And if you happen to have a hi-compression engine with a 4 speed, you cannot even drive it slowly with all that timing.

BTW-2
In all likelihood, your big-cam engine does not like to idle with less that 20 degrees because the AFR is all messed up. After the pick-up is proved working correctly, you will have to sync your transfer slot fueling to your mixture screw setting, and set the idle-speed with timing. Then to get it to a happy quality of idle, you will have to give the engine some by-pass air.
Do not feel that your engine is demanding timing. I have made a 367 (mine) idle just fine at 5* with a 292/292/108 cam, down to 550 rpm. It ran well enough and smooth enough down there, to pull itself on a hard, flat, smooth, surface ; but did not have enough power to climb much of an incline.
But what it does demand, is a reasonably close AFR, that cannot be measured with a gauge, under those conditions.