Knocking 340

Drain the oil, drop the pan, then let it drip overnight.
While yur waiting; pull the sparkplugs, remove the fuelpump and make sure the returnspring is on the arm, and working normally. Check the arm and the eccentric for unusual wear.
Next day; flip the engine upside down. Find a piston that is half way to the top and the crank is at about 90*to the cylinder axis. Put a bar on the balancer, and turn the crank back and forth a couple of degrees each way. The bearing clearance spec is about .002/.003; So the rod should follow the crank immediately with no noise. Repeat until all eight have passed the test. If you find a loose one, pop the cap for an inspection. Start with the drivers side front two which are the two I have seen go out first..
Simultaneously while doing that. Listen for a wrist pin going tappa-tappa on the switch over from CCW to CW.

As the piston skirts come to the bottom of the bore, rock them in the bores, first in line with the wrist pins; there should be very little movement; and NO rocking that does not simultaneously involve the rod.You may have to jam the rods apart to immobilize them.

Then at 90*, in the thrust direction. With those KB243s, there should be very little rocking; skirt clearance on those is .0015 to .0020 for NA street use. If you can get a .004 feeler or more, in there; expect noise.
I put my KB107s (same style piston, built for a 360) in a lil loose, and first start on cold-mornings, I can occasionally hear them.

See what you can find.