[chris vickery]

Grant. From the research I have looked at on US websites, the GM casting number 3769717 indicates a 1958-62 261cid truck engine.
The heads on the Kijiji one are 3836848 are 235cid heads, a common swap in the day, especially for hotrod guys looking to increase compression ratio.
While not a total swap, yes some creativity would be required; short water pump and adapter to relocate it. A few other bits.
As far as the choke, not unusual for the original Rochester B to be swapped out over the years. Matter of fact, many 261s actually have 235 carbs and most guys don't even know the difference or notice it at all.
For the difference in HP unless you are a total purist, a 261 is an ideal swap into a CMP or any other underpowered GM of the era.
The 261 was also well known in the hot rodding circles of the era.
Grant, the Canadian Pontiac 261 came stock with '858 heads. I can see how it may be deceiving. Your block is a '9717 casting?
This surely needs some more investigation as your points do bring up a couple points