Exactly. If it's pump fuel a white plug isn't far removed from normal. Should just be a little bit of color down at the bottom of the insulator which you need a good magnifier light to see properly. I have a hard time seeing plug gap or the cam causing it to just drop dead at 5600. Yeah the performance may drop off but it still should pull to 6k even if it's not making good power. I'd think it's more likely running out of fuel, like when the fuel system isn't keeping up. It doesn't always pop when that happens. When I had my old 360 in the Cuda the fuel pump shot craps on me and it nose dove above 5000 and never popped a lick. I knew it was a fuel delivery issue cause I had a A/F gauge on it and right when it dropped off the gauge went max lean. New fuel pump did the job.
Either that or the MSD has a 5600 rpm pill in it and it's shutting it down.
The carb I have on my 408 sounds like the exact same build as yours and I have 76 jets in the front and 85 in the back so I'd think yours is probably pretty fat with 80's and 85's. What high speed air bleeds does it have?
