Classic/Antique Car Repair: 1969 ford ranchero, 1969 ford ranchero, vacuum leak

Question
what could make cyclenders 5, and 6 on an inline 6 300 engine not fire i cheaked the compression an the plugs are getting a spark im just stumped????

Answer
The first thing that I would do is to take a compression test. It is possible that the head gasket has blown between those two cylinders. The next thing to check is for a vacuum leak in the intake manifold between those two runners where it bolts to the head. If there is a vacuum line near these two cylinders remove it and plug the opening. If the engine smoothes out the device that line goes to could be leaking vacuum.