I introduced myself and my Belvedere in "Slixers". Please excuse the wordiness of this post but I want to be as descriptive as possible.
My '65 has less than 113,000 original miles. When I bought the car a few months ago, it ran great and was really quiet. I changed the oil and didn't notice anything unusual about the old oil. It was a little dirty but not worth concern. A few hundred miles later, I started noticing blow-by from around the oil cap and pcv valve. Just for good measure I changed the pcv valve and no change in the blow-by. I knew some possible causes could be rings or valves. For starters, I was hoping for sticky valves so I took the valve cover off. Everything looked clean and all of the valves appeared to be moving freely. I went ahead and took the rock assembly off, checked the pushrods and cleaned everything. It all looked good so I assembled everything back together and still had the blow-by. My plan was to use a good engine flush followed by some good oil additive. Then problem number 2 started.
For the last couple of days, I though I had been hearing a knocking sound from within the engine. With the valve cover off, it was more noticeable. It sounded like a rod bearing and was confirmed when I pulled the plug wire off of the number 4 cylinder. Crap!
I pulled the pan off and was struck by how clean it looked at the crank. I wiggled each rod and noticed a very slight front to back movement but nothing else. I removed the cap from the offending rod and noted how easily the bolts can lose. I loosened the rod next to this one and it required much more force. The bearings from the noisy rod were not loose or damaged. There was slightly more wear to one side but that was all. There were also a few mild scratches from possible contamination, yet everything looked good. It's worth noting that the bearings have been replaced before by standard size Federal Mogul ones. The journal looks to be in great shape with no knicks or scrapes.
Finally, the conclusion! I'll tell y'all my theory and y'all can let me know if I'm delusional. I'm hoping both problems are mostly unrelated. If they are, perhaps it is from the car sitting for a long time. Since the bearings have been replaced before and that rod cap's bolt's loosened up easily, I believe that the bolts were not properly torqued. My plan is to replace all of the rod bearings and torque them to the right specs. I'll also check the torque of the main caps. If the knocking is gone, I try a good engine flush, like Seafoam, through the intake and fuel line to see if this takes care of the blow-by. If I still see no results, I put some in the crankcase.
Lastly, if I'm still not seeing the results I hope for, I'll start diagnosing the problem to determine if new rings are needed or just a head job.
Does this sound like a good plan?

Measure the crank journals and look carefully at how round thay are... and their size.
Be sure the rod "big-end" bores are also clean smooth and round.
Do the math, you want .001 to .002 of oil clearance. Any excess clearance can be reduced by using the correct bearings.
I give the end face of the rod caps a light sanding, on a hard, flat surface, just to be sure thay are completly clean and flat.

If the crank "pins" are round and smooth and the replacement bearings are correctly "sized"... and the rod cap nuts torqued to spec, everything should work-out fine.

To simply re-assemble with another set of standard bearing... is "risky".
If you elect to do that, at least use some plasti-gauge to check the oil clearances.
DD

Welcome here! I'm with Doc, I think your plan is risky not only in terms of time/effort, but also in terms of engine health. Don't guess or assume…measure!

As for engine flush: Careful, you can easily "flush" an engine right down the toilet. See soup for a safer way, but it doesn't sound like your engine is internally dirty since you're talking about things being nice and clean inside.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks for the advice gentleman.

So Doc, to make sure we're on the same page, I need to take a micrometer and measure at different points on each journal to verify that they are properly round. Then measure the connecting rods *with cap and bearings* and see what kind of difference I have. If the clearance is greater than the specs you gave me then I’ll need the corresponding bearing size to make up from the difference. Is that right? I've played around with engines most of my life but this is the "deepest" I've ever gotten into one.

And SlantSixDan, I'll check on the soup. I mainly want to concentrate on trying to clean the valves for buildup along the ends or around the seals. That's why I was considering the Seafoam down the carb or through a vacuum line. If the soup does the same thing in a safer manner, then that may be the way to go. Of course this is with the hope that this will cure the blow-by.

Crispy valve stem seals won't cause blowby, but they'll cause oil consumption. See here for info on replacing valve stem seals.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

That is correct.
With the con rods, you will need to torque the cap to the rod before taking those measurements but even then... that will be a hard measure to accuratly get with the engine still in the car, with the pan off.

FYI that replacement bearings do come in .001, .002, etc. sizes so it is possible to reduce excess clearance IF the journal is still round.
DD

That is definitely one worry I have, trying to measure the rods while the pistons are still in the engine. It may be possible to do this if I rotate the crank journal for each cylinder to the side.

However, I wonder if there is another option. If I measure the journals and they are still round and within the factory specs (one website says 2.1865 to 2.1875 while another says 2.189), can that measurement alone be good enough if I'm using new bearings or is there a chance that the rods (even with the new bearings) could be out of spec when torqued?