Engine stalls when you pull number 5 spark plug wire.

To return to the full CNC Speedmaster heads. The exhaust ports are a lot larger than the doug headers I use. I had to rebuild the exhaust gasket and give the sharp surfaces a little more radius to create a normal path. But it is certainly not optimal. and I don't understand why those exhaust ports have to be so big. Exhaust fumes are now hitting a wall. Could intake reversion also have something to do with it?