help annoying pop in exhaust

Well since youve been able to isolate the cylinder, Ive got a test for you that can prove the exhaust valve is or isnt hanging up.
It goes like this; Get the engine running on the passenger side cylinders and #1 only. Then put a section of cerealbox ,or similar stiff but a tad flexible material,against the driver side collector. If that posterboard gets sucked up against the flange after every pressure pulse that would be the piston sucking it there through an open exhaust valve. Bad news.
I have never done this on a V8.You may be the first guy on the planet to try. It works really well on small engines, with instant verification..
To disable numbers 3,5, and 7 which all dump into a common collector, I think the best way would be to remove the spark plugs, and to short the plugwires to ground.Otherwise the header-pulse tuning will give an erroneous reading.Yeah it will be noisy. The curb idle screw will need to be cranked in to keep the engine running. Dont forget to back it out later.
I guess it would be wise to have a fire extinguisher handy. There may be some fuel being expelled from the open cylinders.Ive run one open like that now and then, but never three.The one open didnt expel fuel at idle.
Or you could just drop the header and check the clamp-pattern.I think I said that one time already.
Ok so good luck.