Dead Cylinder Below About 1500 rpm's

How do you know it is in #6? 130 psig in all cylinders sounds great. Strange that someone assembled the engine with the wrong size piston rings. Any idea how that happened? The outer cylinders tend to run leaner, so would be first to suffer "lean mis-fire". Could also be the spark wire or something in the distributor cap. People have found many caps w/ the tower circle off center. Slant Six Dan has posted the best PN to buy.