With a carbureted engine the only way you cannot be getting fuel to only two cylinders is to have something keeping those two intake valves from opening (cam lobes flat, pushrods bent/broken, rocker arms damaged, valve springs, valves) unless you have a couple of dead mice in the intake, and you should smell that by now . You say you checked for spark at each cylinder - how did you do that, and was the spark a good, hot blue-white spark at each one? How about switching plugs, using two that you know were firing in other cylinders and see of your miss follows the two plugs you had in 1 & 3? Could be as simple as bad plugs out of the box? Next I would look at the distributor - what type ignition are you running?