I would be tapping the smaller hole. The larger one is the dowel pin hole so it cannot be touched because I need to put my new dowel pins in there of course.I'm seeing something I didn't realize before. I thought you had drilled the same diameter as the dowel. You have a small hole at the bottom of a larger hole. Are you going to tap the large hole or the small hole?
The small hole looks like it may not have enough wall depth for a tapered thread. And the large hole doesn't seem to have enough depth for even a bottoming tap.
As far as wall depth, if I understand you, I measured it at about 0.450". That's from the back of the dowel pin hole to the edge of the oil passage. To put it another way, this is the maximum length of the threads I would cut, or the maximum length of the plug. Is that enough for tapered threads?Fortunately, I have my engine out and apart right now so I could go have a look.
The bottom hole on the oil filter adapter, the one that you drilled into, is the pressure side of the oil pump before it goes into the filter. You can see in the 5th picture you posted that shows the back side of the block, that passage makes a 90 degree turn and goes down to the oil pump. The upper passage across the back of the block is the one that comes out of the filter and goes across to the lifter gallery, "output" from the filter. So, that hole you drilled is going to "see" the maximum oil pressure that the pump puts out.
True, that's a small hole and very shallow to be able to cut a tapered thread into for a small pipe plug, but I think the good news is that I don't think it HAS to be a tapered pipe plug to work. That's because the dowel pin will be sitting on top of it, and that ought to keep whatever you put in there from backing out. If it were me, I'd consider cutting normal threads into that small hole and putting a regular hex socket plug into it, with JB weld packed into the threads to seal it. Once that dowel pin is sitting 'behind' and on top of it, you shouldn't have to worry about it backing out, and if you make sure to get both the threads on the hole and the threads on the plug nice and clean, that very thin layer of JB weld that will be left after you put the plug in ought to seal it nicely. Make sure the plug doesn't protrude into the oil passage so as not to introduce a flow restriction. If you go this route, coating your tap with heavy grease before you cut the threads will help trap the chips so they don't wind up in your oiling system. Also with the pan off and the pump removed, you can blow air into that passage from the oil pump mounting face and that will clear it out. If for some reason you don't get a good seal, it should be obvious because you'll be able to see oil leaking from around that dowel pin after it's all up and running. It wouldn't hurt to use some kind of sealant around the dowel pin, perhaps something like regular old pumber's "pipe dope" for insurance.
My .02 - from someone who's also made more than his share of mistakes and created additional problems for himself therefrom.
A concern I have of cutting regular threads is, what would stop it from "backing in" to the oil passage? I know the dowel pin would stop it from backing out, but would the oil pressure (or JB Weld) -always- be sufficient to keep it from continuing into the oil passage?