You can also try swapping the cabs front to back. You say 5 and 6 didn't change much with plug wires pulled, this leads me to believe the back half of the engine is lean. The way the firing order goes, 142635, it's unlikely the distributor is jumping number 6 and 5 and still have 3 working.
Try closing the choke on the rear carb or both of them.
If the engine ran before the swap, it still should run, valves and such don't go bad just setting. On the other hand, if it ran marginally before, the duel setup will just make it worse.
Joe
