73' 340/904 duster racestoration

By looking at the plugs you removed, It appears that one side of the carburetor is running richer than the other. The way the plenums are split in the intake manifold, it feeds 2 cylinders on one side and 2 on the opposite side. Look at the pictures of the plugs, inner 2 and outer 2 are carbon fouled.
I haven't worked on edelbrock carbs, but when you get it going and it's idling, look down the carb and see if one side is dripping fuel.
This could explain the bad fuel mileage also.
Have you pulled the plugs again, after it wouldn't start, to see if it is flooded?