I know you say you cleaned the fuel system, does this include stripping the carbs, checking any diaphragms for holes, and 'O' rings to spindles?
When you checked the float heights did you remove and check the float needle valves are all free to operate?
You state that all ingintion leads and caps are originals and are OK.
How did you test them?
Are the leads silicone cored or copper cored?
What resistances are you getting during testing?
Are you positive that they are working under cyclinder pressures. Have you used the substitution method of known good leads and caps to cylinders that are not firing OK?
When you checked for valve operation did you check to see if the valve springs had weakened due to being under continuous pressure?
Were the inlet and outlet tracts closed off to the atmosphere during this prolonged layup? Possible build up of corrosion on valve edges and or seats caused by atmospherics being able to get into the engine internals.
What compression reading did you get and did you carry out the test with the throttle open or shut?
Also did you do dry and then wet compression tests?
That's just a few of the possibles off the top of my head at the moment. Hope some of these will help you locate the problem.