Need Help Diagnosing Engine Knock

Everything is a guess, and after you find out what it is, it can be easy to go back and make it fit the symptoms, but hard before the fact to differentiate between possible causes based solely on the noise.
Rod/main bearings is likely the worse possible problem that could cause a knock, so I would want to eliminate that as a possibility as soon as possible. It would also be the hardest to repair if left to run until "un runable". If cought early and the reason remedied, the engine can be restored with new bearing of the same size.
An engine will run a long time with piston slap and have no real harm, but better to re size the piston early if you are going that route.
Some engines can feed oil to other locations from the cam bearings, and if they aren't orientated correctly can cause oil starvation down stream. However it is most often top end lubrication that is fed off the cam bearings.
It is also possible that poor mfg/ hardening of the tappet could be the cause and nothing about the rebuild could have caused the noise.
I think it will reach the point that dis assembly will be needed to nail it down and repair. Disappointing, but it may very well be no fault of anyone involved other than the mfg of the parts. It is rare, but not unheard of. I had to re-do one cyl on a K-19 Cummins due to inferior parts on that one "hole".

Cam bearings I do know were oriented correctly and matched runs in the block. Lifters were new, as was the cam. Cam may have been a regrind (memory fades), and it sure could be wear on either of those.

Oh, BTW, I once diagnosed a bad crank from the noise coming off the crankshaft, sold the job of changing out the crank, and got the truck in the shop. 1st order of business was to pull the (auto) trans. when I went to un bolt the convertor, I found the bolts were loose and had worn the bolt holes in the flex plate oblong. I sure was red faced when I called the customer, but he was very happy. Instead of a crank replacement, all he needed was a flex plate replacement. I thanked me for my honesty, as he said I could have replaced the flexplate and changed him for the crank and he wouldn't have been the wiser, but not the way I do business.
Not likely with a manual trans as the clutch will load the assembly endplay, but could be your problem if the knock goes away when you step on the clutch.

I saw your post on that earlier and wondered, but clutch in or out, and driving or sitting still doesn't make a difference. I've even been wondering if there are maybe two gaps on the #6 intake - the gap between the rocker and valve, and a gap between the lifter and cam. That maybe the lifter just doesn't drop all the way in the bore, but the cam lobe hits it on the way up, and that the feeler gauge just takes out one of the gaps so that the valve spring on closing pushes the lifter down against the cam. Pure speculation on my part. I am pretty sure I wasn't holding the valve open at all with the gauge inserted, as it was a hair thinner than the gap.

Biggest concern of course is rod or wrist pin, as I'd hate to have a rod go through the block and find out that way. SO the next order of business will be to pull the pan and see.

In the days of the SD-220, 100k was considered the useful life of an engine between rebuilds. So 80k would be 80% of that...

Long-distance diagnosis is an inexact science, but 80,000 miles is sufficient to develop piston slap. If the machine shop checked piston-to-bore clearances, they may have simply missed checking one cylinder...or not checked the lower end of the bore. Lots of room for human error. It happens.

Oil has changed a lot since the 50's as had what is considered acceptable running. Today we don't put up with a mis fire for days or months.
I had a '65 vintage engine (225 Dodge) that I ran until 380,000 miles. I did update to electronic ign and did re-ring it somewhere around 300,000 miles. 80K today is nothing, esp if it was running well.
Sometimes thing just fail, you can write books as to why, but it doesn't change anything. I recently had a major failure on my 855 Cummins. Was running under load and wasn't using oil or showing any other signs, when it just blew. I haven't dis-assembled to find out why yet, it doesn't change the fact that it let go.

While everyone has potentially valid solutions, I re-read the original description and it seems to me the problem might and I’ll reiterate MIGHT be in the valve train. The noise went away with a feeler gauge inserted between the rocker and valve so there is something in that section of the valve train that is contributing to the noise like, and I assume you have hydraulic lifters, a sticking lifter or a worn push rod. I believe I read that you said it is occurring on every other revolution of the crank meaning on either an ignition stroke or exhaust stroke. The other guys who have offered solutions have had their arms elbow deep into engines far more times than I have so it would not surprise me if I was wrong and they right. Just offering up my 2 cents worth. Many years ago lots of shade tree mechanics used Marvel Mystery Oil and other concoctions to free up sticky valves and valve train components. It may be silly to suggest, but if something like that would help, would be far simpler than tearing down the engine.

Unfortunately it is extremely hard to diagnosis something you can't lay your hands on and even more so if you can't hear the problem.
Given that, there is very little you can check without getting into it. Checking valve train lash (it has mechanical tappets) is one, also checking cam lift and rocker action with a dial indicator is another. Beyond that some dis assembly is going to be required to get to further.
If the lash is incorrect or if there is evidence of cam lift problems, that is about all you are going to be able to verify without pulling things apart, and lash is the only thing you are going to be able to correct without disassembly.
Anything else you can do with the engine together may point to an area of concern, but repair will require getting to where the problem is.

Might try running it with #6 plug wire off and see if the knock goes away. If you take the fire away there would be no load on wrist pin, piston or rod bearing. If knock disappears I would suspect something in that area. Also as cornbinder said, with the feeler gauge installed while running could have lowered the compression giving you the same result. Food for thought.
Dennis

I used a thinner feeler gauge than the gap between the valve and rocker arm, so I don't think it was holding the intake open, and the banging did go away. I checked again, however, and it also went away with pulling the #6 plug wire. So the oil pan came off today. Here are the #6 rod bearings. The journal is smooth and unmarked. They are at least better than I feared, but would like opinions on condition.