Technical V8 60 two dead cylinders

Got a cooling system in the wing tank car so I've been running the engine enough to notice a misfire.I shorted the cylinders on at a time and it's two cylinders on the right, 3 and 4 don't fire. For sure there's spark since I can see it jump to the grounded test wire. I even reversed the plug wires, no difference. Compression test shows all cylinders 75-80 psi on the the third stroke of the gauge needle. No signs of a vacuum leak......
i did replace both head gaskets on this engine,the heads were ok. No signs of water leaking into the cylinder..
The engine has a two carb intake with 81's Ideas before I pull the head? Thanks

 

My first thought how's the plugs? maybe switch with a good plug see if trouble moves .

 

Yeah, no difference. I pulled the intake looking for cracks,none to be seen. Pulled the head and the head gasket showed no signs of leakage. Valves move up and down