47-55.1 Exhaust valve not holding pressure?

[duramaxguy]

I've got a 51 3100 with the 216. beautiful restoration about 15 years ago. Has always run very smooth.

A couple months ago i'm driving up the hill and suddenly the truck starts missing/coughing/surging. Looses all power. Idles with a slight miss.

I confirmed that the fuel system is good. I confirmed that i'm getting good spark on all the cylinders. Confirmed that I'm getting good vacuum on the manifold.

while the truck idles, I short out each plug one by one. All plugs, when shorted, cause the engine to run even rougher, except #3.

What is my next step? My hunch is that the exhaust valve on #3 is either stuck open or the seat has gone bad. What is the best way to confirm? I don't want to pull the head until i'm reasonably sure that it must be done.

Thanks