My 340 getting 6 mpg

I would try another carb if you can. Even a factory carb. Anything. The carb you have may have some sort of internal fault. Fuel is pouring into the engine, almost literally. Have you checked for leaking needle & seats? Correct float level? Those are basics to carb set up. Tuning starts after the correct idle ign timing has been determined & set.
If someone advised you to use 10* initial timing, then never take their advice again because they have no clue.