The blCk powder is likely just carbon dust from unburned fuel now that you've lost 2 cylinders.
First off, do or have someone do a leak-down check on all 8. That's gonna t ol you way more than a compression check. You're problems are way beyond a bad head gasket unfortunately. If you can crank and run the engine with no water leaks, the head gasket is fine.
If you got the motor hot enough twice to burn up some of your oil, you likely burned either a couple valves or most likely trashed some piston rings which are now in little tiny pieces down in the oil pan since you said you saw smoke (I'm guessing blue smoke - oil) in the exhaust.
The leak down test will hopefully tell you where the issue is. When you shove the air in the cylinder if you hear it coming of the crank case vent (PCV) your rings are toast. If you hear it coming out the throttlebody/carb, your valves in that cylinder are trashed.
Good luck with it all, but hey, motor work is fun! Put ya some dome top Pistons in if you have to tear it down and get a couple more ponies outta the ol girl. These engines are nothing more than old school Chevy 350's. There's all sorts of parts out ther to help her out!
If you need any help or have questions feel free to pm me.