plugs look right. The parallel valve engines call for at 1/2" reach plug and this is what you have here. DO NOT put in a longer plug.
Do you have mags? Above 8-10K' mags struggle to produce enough energy to fire the plug and the spark can be weak. LOP mixtures are more difficult to light (gets worse the further LOP you go) than rich mixtures. This leads to a potential situation where you can get mis-fires. Mags that are running well with good wires and plugs don't usually have issues here, but if the system is sub-optimal, this is where problems will often present themselves. You want to pull the mag apart and check the egap as well as for carbon tracking and or wear on the cap and rotor.
If you have an EI setup, the EI and coil have no issues with altitude or LOP, but could be a sign of a problem with wires or a bad plug reducing spark energy.
Misfiring only when LOP and only at high altitudes points to marginally low energy being produced by the ignition source or delivery system.
EDIT: Sorry, now see that you have EI. I would be looking at each plug wire. Pull each wire off the coil and check resistance to ground. They should all be within about 2K ohms of each other. Longer wires should have greater resistance. If you find an outlier, pull the wire off the plug and check all of the plugs and wires separately to identify cause.
Larry