OK, I can't see the pictures that clearly. Do you have little "divots" in the pistons that look like they were shot with a shotgun using bird-shot in it? If so, that's detonation & is caused mainly by running 87 octane gas when you need premium, but can also be caused by excessively advanced timing & a few other things. I see that those are not stock 74 318 pistons, so that is my guess.
The MAIN thing to be concerned about if you had detonation is the top piston rings which can break & destroy a motor...divots in your pistons aren't good, but rings are critical. I suggest you drain ALL the gas, put 92/93 octane it in & see if you knock goes away.... and keep running premium forever.
On the other hand, if your pictures are showing black BUILD UP on the pistons, then buy a can of Sea Foam, pour it down in the carburetor while it's running. The engine will stall & you let it set like that for 15-30min. Then start it up & the engine will smoke like crazy. Rev the engine slightly to clear most of the smoke coming out of the tailpipe, let the engine warm up & then drive it aggressively for a few blocks...nail the throttle 3-4 times. That will clear the carbon off the pistons.
Having said all that.... after you put in premium, if you still have a piston/rod knock, then don't do any of this & take a look at rocker arms & pushrods. If all looks good then take a look at the main/rod bearings in the engine
