Since you have two adjacent cylinders that have good compression and aren't working the most obvious problem may be wires crossed. That could also account for the clean plugs, they get washed with gas but never get lit!
As for the low comp on 6, it could be a burned or stuck valve or a badly damaged piston/ring assembly. My money is on a valve. As someone else pointed out , air in the spark plug hole with both valves closed will tell you quickly. Air leak heard from the carb= intake valve, From the tailpipe=exhaust. With the valve cover removed you can see if the valve is seating.
