Thanks for the quick replies. The car idled well (actually better than with the other cam). It was idling evenly with no noticable miss. But I did not run it long after I started hearing the noise, the noise was pretty loud.
Oil pressure was good - steady 50 psi at idle.
After closer examination of the damaged plug from number 8 I noticed the porcelain insulator is broken and the whole insulator slides up and down on the electrode. Could the cylinder have been dead and firing in the exhaust? You think I would have noticed a miss though.
I cycled the engine with a remote starter switch while the plug was out to see if I could hear anything rattling around in there - nothing. It sounds perfect when just cranking. I also put the cylinder close to TDC and fished around with a piece of electrical wire through the spark plug hole, could not seem to find anything inside the cylinder.
I don't have a compression gauge handy, but the cylinder seems to have good compression when I cycle it with my thumb over the plug hole. At least similar to the others.
I'll try putting a new plug in #8 and my original carb back on and see if it still bangs. Then off comes the header and I'll look in the exhaust port with a flashlight.
My gut feeling tells me something is wrong in the bottom end though - conn rod coming loose and smacked the plug maybe? I hope not, all top notch components down there (KB pistons, Eagle 5.7" SIR rods, Eagle cast crank, ARP bolts, studded 4 bolt main).