Hmmm, for my '76 TNT with the type 343 you're supposed to use an 8something.
If it ran good for 3 hours, then lost midrange, then wont start at all, AND the plugs are black.... Hmmm,
Do you get spark at all?
If you take the plugs out, put them back on the wires, they them across the engine so the metal part of the plug contacts the metal part of the engine(important) and crank it over, can you see spark across the sparkplug gap?
What color is it? Weak and yellow would indicate a bad coil for that side, bright and blue would be good.
I'm going to lean toward still saying its a carb problem, but the spark test will tell alot. Don't replace your coils until you've tested.