Can't find misfire

Im pretty sure that the valve clearances should be ten and twenty. I think your exhausts are too tight.I would back them up first.If theyre too tight, combustion pressure can upset the intake vacuum, and subsequently mess up the low speed carburetion .So thats #1 on the to-do list.
However, since this problem existed before the valve adjustment I think theres more to it. I believe that another problem exists and it is in the lowspeed circuit.I have a feeling your float level is too low, and in an effort to keep it running, the idle speed was increased to 800rpm. This puts the T-blade too far up the transfer slot, and it may run dry. This could be aggravated by a plugged idle air bleed. I would be almost undriveable if the timing was stock. But at 10*, advance its helping.
So the next thing to do is set the float level. And while the top is off blow out the airbleeds. Then after its back together see if you can bring the idle speed down some.They usually idle sweetly at 650ish.
I know your not too interested in resetting the valves.But sometimes you gotta back up to go forward.
I suppose theres no harm in doing the carb work first. I just know .012 is too tight and if they continue to run there,say at highway speed, they will overheat and burn. Then you will be spending money that you also dont want to do.And after years and years and years of doing this kind of work, I have found that my time and therefor the customers bill, is best served by starting with compression.But since you dont have a compression tester, we can at least make sure the valves are closing. Later, if Im wrong about the float level/airbleeds, and the engine fails to respond, then a compression test will be required.
So, happy Easter