First of all, THANK YOU for all of your assistance and time spent looking over old cast iron.
After jacking the front end up and spending some quality time applying elbow grease, I have some new / accurate info to go off of in addition to the bump-2 casting at the driver's side front. No, there was no hidden 352 or mirror 105 casting hiding unfortunately.
I do not seem to have any cross bolt main bosses whatsoever.
The block doesn't seem to have the ribs on it as I have seen elsewhere.
The pass. side has an upside down "4" toward the front of the block, down low. There is a threaded hole for the trans coooler lines, and just behind that, next to the large starter, is a 1" deep threaded hole as you inquired about.
The driver's side has a 3B18 (Feb. 18th '63 matching the intake) casting near the oil filter, as well as a large "V", and another upside down "4" up higher on the block, toward the front. There are no other threaded holes on the driver's side, just the petcock that sits up higher off the oil pan and toward the middle-rear.