Guys - how do you get a very dirty plug with a blown head gasket? There are several other choices that are easier to work with and more likely to be the root cause. They have been described above.

A 300hp crate motor has a carbureted intake. Any time air/fuel mixture flows through a manifold there will be unequal fuel distribution. So your #8 could have a river of fuel running down the bottom of the plenum into the runner. I have an old Direct Connection engine manual that details how to epoxy popsicle sticks into the plenum to help even out the mixture.

R.

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.

Runners

That is the intake manifold I’m running.
Part of the confusion on the float level is holley reccoemnds the fuel to be at the bottom of the sight glasses, quick fuel reccoemnds having the fuel half way up the sight glasses.
So I did have the float level at the bottom of the sight glasses, I noticed when it was cold out especially that it was pretty lean at part throttle.
Specially after I slowed my advance rate down in my distributor.
So I raised the floats to half way up the sight glasses and that solved the lean spot at part throttle and made the engine feel like it was running better, specially when it’s cold.
So I’m tninking since I raised the float level to what quick fuel reccoemnds that I probly
Should increase the idle air bleeds to lean out the idle and off idle circuit a little bit.

Or maybe lower the floats back down a little bit so they are 1/4 of the way up the sight glasses instead of half way up.
I just figured I’d set the float level to what is specified by quick fuel since it is a quick fuel carb.