Rod knocks have a tendency to create a double-tap noise when a spark plug is disabled. Main bearings are more likely to cause a lower frequency "thump" type knock that doesn't change with or without spark. A main bearing knock will also make a "thump- thump- -thump" noise during acceleration that goes away or diminishes noticeably once a steady speed is achieved. I've seen several engines get an "in the frame" rebuild over a noisy fuel pump. A broomstick, dowel rod, or a long screwdriver held against the pump housing at idle will usually pinpoint the noise pretty reliably. Try removing the fuel pump, blocking off the hole in the block, and running the engine on a gravity fed fuel tank.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!