I am trying to offer ideas to the OP.
Without being there to check other signs it is admittedly hard.

If compression is low in #8 there will be a missfire and unburnt fuel,
or if there is induction tiggered missfire in #8 from #6 ignition wire,
in either instance i would expect to see a gasoline wetted sparkplug.

I would expect if the manifold distribution is really bad to # 8
then the symmetrical other runner of the M1 dual plane would show at least some sign of bad distribution, but a really badly malfunctioning carb can do unpredictable things.

A video boroscope with a 9 mm camera head to go through the sparkplug hole and also down intake runners could narrow this long distance troubleshooting down. These are value priced now days.