I have a theory. If that cylinder is glowing red when running, it could mean the exhaust valve lash is too tight, possibly not fully closed during firing and sending hot combustion into the header tube. Maybe the combustion event has enough pressure to "shut" the exhaust valve , slamming the valve against the valve seat every combustion event. That could explain the knocking increasing with revs. If you pull the plug on that cylinder and the knocking/glowing exhaust goes away, it means there is no combustion pressure to knock the valve against the head, and no heat to blow the tube. That could also explain rod knock, though.
Try adjusting rocker lash first. Many guides on the internet on how to do that to a SBC. Get that cylinder to "base circle" of the cam (valves closed) by barring the engine by hand. Loosen the rocker nut, then tighten by finger. When rocker nut is finger tight, go another 3/4 of a turn, maybe air of the side of less turn for Cylinder 6. Best of luck dude.
